structural effects

Structural EffectsStructural Effects

Ms. Anjelyn del Rosario

Chemistry 31Chemistry 31

UNIVERSITY OF THE PHILIPPINES MANILAPadre Faura, Ermita, Manila

SS, 2009 – 2010

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Types of Structural EffectsTypes of Structural Effects

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A.A. Electronic EffectsElectronic Effects - affect electron distribution in a molecule- affect electron distribution in a molecule

Electron release or Electron-donating Electron release or Electron-donating – designated by a plus – designated by a plus (+) because it increases electron density in certain parts of (+) because it increases electron density in certain parts of the moleculethe molecule

Electron withdrawal or Electron-accepting Electron withdrawal or Electron-accepting - designated by - designated by negative (-) because it decreases electron density in certain negative (-) because it decreases electron density in certain parts of the moleculeparts of the molecule


B.B. Steric EffectSteric Effect - generally associated with the presence of bulky - generally associated with the presence of bulky

groups resulting to crowding or congestion in a groups resulting to crowding or congestion in a molecule.molecule.

1.1. Pi electron delocalization or ResonancePi electron delocalization or Resonance

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a. Electron Release Through Resonance, +Ra. Electron Release Through Resonance, +R present whenever an atom with at least one lone pair is present whenever an atom with at least one lone pair is

bonded directly to benzene ring.bonded directly to benzene ring. Groups exhibiting +R:Groups exhibiting +R:

- OH, -OR, -NH- OH, -OR, -NH22, -NHR, -NR, -NHR, -NR22, -X (Br, Cl), -X (Br, Cl)

Electronic EffectsElectronic Effects

.. .. .. .. .. .. .. .. ..:

ππ electron delocalization electron delocalization enrichesenriches the ortho, para the ortho, para positions of the benzene ring.positions of the benzene ring.

The ortho and para positions are likely to be attacked by The ortho and para positions are likely to be attacked by electron-loving reagents.electron-loving reagents.

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b. Electron Withdrawal through Resonance, -Rb. Electron Withdrawal through Resonance, -R Present whenever the substituent has an atom that is Present whenever the substituent has an atom that is

multiply bonded to a more electronegative atommultiply bonded to a more electronegative atom Groups exhibiting –R:Groups exhibiting –R:

-NO-NO22 , -C(=O)H, -C(=O)R, -C , -C(=O)H, -C(=O)R, -CΞΞN, -SON, -SO33H, -C(=O)OH, H, -C(=O)OH, -C(=O)OR, -C(=O)NH-C(=O)OR, -C(=O)NH22

The effect of The effect of ππ electron delocalization in this case is to electron delocalization in this case is to depletedeplete the electron density at ortho and para positions. the electron density at ortho and para positions.

Ortho and para positions are likely to be attacked by Ortho and para positions are likely to be attacked by nucleus(+) –loving reagents.nucleus(+) –loving reagents.

1.1. Pi electron delocalization or ResonancePi electron delocalization or Resonance

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Involves delocalization of sigma electrons from C-H bond of Involves delocalization of sigma electrons from C-H bond of the spthe sp33-s type, that has a neighboring atom with p orbitals-s type, that has a neighboring atom with p orbitals

there is considerable overlap between spthere is considerable overlap between sp33 orbital (75% p) and orbital (75% p) and unhybridized p (100% p).unhybridized p (100% p).

Does not take place as readily as Does not take place as readily as ππ electron delocalization electron delocalization The greater the number of contributing structures, the The greater the number of contributing structures, the

greater the influence on greater the influence on δδ electron distribution. electron distribution.

2.2. Sigma electron delocalization or HyperconjugationSigma electron delocalization or Hyperconjugation

Stabilization of CarbocationStabilization of Carbocation

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a.a. Electron Attracting Inductive Effects (-I)Electron Attracting Inductive Effects (-I)i.i. Groups with excess (+ ) chargeGroups with excess (+ ) charge

R-N-RR-N-R -S-R -NH -S-R -NH33++ -NO -NO22

++ R RR R ii. Electronegative atoms/ groupsii. Electronegative atoms/ groups -NH-NH22 , -OH , -NO , -OH , -NO22 , -OCH , -OCH33 , -F , -O , -N , -Cl , -Br , -F , -O , -N , -Cl , -Br iii. Orbital Electronegative groupsiii. Orbital Electronegative groups

-C-CΞΞCH , CH , -C-CΞΞN , N , -N-NΞΞN , phenylN , phenyl

iv. Polarizable outermost electrons iv. Polarizable outermost electrons -I-I

3.3. Inductive EffectInductive Effect Result in distortion of electron cloud because of the presence of Result in distortion of electron cloud because of the presence of

strong electronegative groups or atoms, electron-repelling strong electronegative groups or atoms, electron-repelling groups that have highly polarizable outermost electrons.groups that have highly polarizable outermost electrons.

++ ++

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b.b. Electron- Repelling Inductive Effects (+I)Electron- Repelling Inductive Effects (+I) Exhibited by alkyl groups and those that possess excess Exhibited by alkyl groups and those that possess excess

negative chargenegative charge

i.i. Alkyl GroupsAlkyl Groupsii.ii. Negative GroupsNegative Groups

-COOH , -S: -COOH , -S: -- , -O: , -O: --

3.3. Inductive EffectInductive Effect Result in distortion of electron cloud because of the presence of Result in distortion of electron cloud because of the presence of

strong electronegative groups or atoms, electron-repelling strong electronegative groups or atoms, electron-repelling groups that have highly polarizable outermost electrons.groups that have highly polarizable outermost electrons.

.. .... ..

.. .... ..


4.4. Steric EffectsSteric Effects These are structural effects as a result of the presence of bulky These are structural effects as a result of the presence of bulky

groupsgroups Bulky groups result in congestion in the moleculeBulky groups result in congestion in the molecule Congestion is relieved by twisting of bonds Congestion is relieved by twisting of bonds

groups will be forced out of plane.groups will be forced out of plane. electron distribution is affectedelectron distribution is affected

Experimental Observations Experimental Observations and and


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1.1. Physical PropertiesPhysical Propertiesa.a. Dipole MomentDipole Momentb.b. Boiling PointBoiling Pointc.c. Melting PointMelting Pointd.d. SolubitySolubity

2.2. Chemical PropertiesChemical Propertiesa.a. AcidityAcidityb.b. BasicityBasicity

Intermolecular ForcesIntermolecular Forces• Intermolecular forces Intermolecular forces are also referred to as noncovalent are also referred to as noncovalent

interactions or nonbonded interactions.interactions or nonbonded interactions.

• There are several types of intermolecular interactions.There are several types of intermolecular interactions.

• Ionic compounds Ionic compounds contain contain oppositely charged particles held oppositely charged particles held together by extremely strong together by extremely strong electrostatic inter-actions. These electrostatic inter-actions. These ionic inter-actions are much stronger ionic inter-actions are much stronger than the intermolecular forces than the intermolecular forces present between covalent present between covalent molecules.molecules.

Introduction to Organic MoleculesIntroduction to Organic Molecules

• Covalent compounds are composed of discrete molecules. Covalent compounds are composed of discrete molecules.

• The nature of the forces between molecules depends on the The nature of the forces between molecules depends on the functional group present. There are three different types of functional group present. There are three different types of interactions, shown below in order of increasing strength:interactions, shown below in order of increasing strength:

van der Waals forcesvan der Waals forces

dipole-dipole interactionsdipole-dipole interactions

hydrogen bondinghydrogen bonding

Intermolecular ForcesIntermolecular Forces

Introduction to Organic MoleculesIntroduction to Organic Molecules

• van der Waals forces van der Waals forces are also known as London forces.are also known as London forces.

• They are weak interactions caused by momentary changes in They are weak interactions caused by momentary changes in electron density in a molecule.electron density in a molecule.

• They are the only attractive forces present in nonpolar compounds.They are the only attractive forces present in nonpolar compounds.

Even though CHEven though CH44 has no net has no net

dipole, at any one instant its dipole, at any one instant its electron density may not be electron density may not be completely symmetrical, completely symmetrical, resulting in a temporary resulting in a temporary dipole. This can induce a dipole. This can induce a temporary dipole in another temporary dipole in another molecule. The weak molecule. The weak interaction of these temporary interaction of these temporary dipoles constituents van der dipoles constituents van der Waals forces.Waals forces.

Intermolecular Forces – van der waals Intermolecular Forces – van der waals


• All compounds exhibit van der Waals forces.All compounds exhibit van der Waals forces.

• The surface area of a molecule determines the strength of the van The surface area of a molecule determines the strength of the van der Waals interactions between molecules. The larger the surface der Waals interactions between molecules. The larger the surface area, the larger the attractive force between two molecules, and the area, the larger the attractive force between two molecules, and the stronger the intermolecular forces.stronger the intermolecular forces.



• van der Waals forces are also affected by polarizability.van der Waals forces are also affected by polarizability.

• PolarizabilityPolarizability is a measure of how the electron cloud around an atom is a measure of how the electron cloud around an atom responds to changes in its electronic environment.responds to changes in its electronic environment.

Larger atoms, like iodine, Larger atoms, like iodine, which have more loosely which have more loosely held valence electrons, are held valence electrons, are more polarizable than more polarizable than smaller atoms like fluorine, smaller atoms like fluorine, which have more tightly which have more tightly held electrons. Thus, two Fheld electrons. Thus, two F22

molecules have little molecules have little attractive force between attractive force between them since the electrons them since the electrons are tightly held and are tightly held and temporary dipoles are temporary dipoles are difficult to induce.difficult to induce.



• Dipole—dipole interactions Dipole—dipole interactions are the attractive forces between the are the attractive forces between the permanent dipoles of two polar molecules.permanent dipoles of two polar molecules.

• Consider acetone (below). The dipoles in adjacent molecules align Consider acetone (below). The dipoles in adjacent molecules align so that the partial positive and partial negative charges are in close so that the partial positive and partial negative charges are in close proximity. These attractive forces caused by permanent dipoles are proximity. These attractive forces caused by permanent dipoles are much stronger than weak van der Waals forces.much stronger than weak van der Waals forces.


Intermolecular Forces – Dipole-dipole InteractionsIntermolecular Forces – Dipole-dipole Interactions

• Hydrogen bonding Hydrogen bonding typically occurs when a hydrogen atom typically occurs when a hydrogen atom bonded to O, N, or F, is electrostatically attracted to a lone pair of bonded to O, N, or F, is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule.electrons on an O, N, or F atom in another molecule.


Intermolecular Forces – H-bondingIntermolecular Forces – H-bonding

Note: Note: as the polarity of an organic molecule increases, so does the as the polarity of an organic molecule increases, so does the strength of its intermolecular forces.strength of its intermolecular forces.


Intermolecular Forces – H-bondingIntermolecular Forces – H-bonding

Dipole MomentsDipole Moments

Compound Dipole Moment

Compound Dipole Moment

1. Ethyl chloride 2.05 D 7. Methanal 2.37 D

2. Vinyl chloride 1.44 D 8. Propanal 2.73 D

9. Methyl bromide 1.79 D

3. Benzene 010. Ethyl Bromide 1.88 D

4. Nitrobenzene 3.95 D 11. Isopropyl bromide 2.04 D

5. toluene 0.37 D

6. 2,3,5,6- tetramethylnitrobenzene

3.62 D

Physical Properties—Boiling PointPhysical Properties—Boiling Point

• The The boiling point boiling point of a compound is the temperature at which of a compound is the temperature at which liquid molecules are converted into gas.liquid molecules are converted into gas.

• In boiling, energy is needed to overcome the attractive forces in In boiling, energy is needed to overcome the attractive forces in the more ordered liquid state.the more ordered liquid state.

• The stronger the intermolecular forces, the higher the boiling The stronger the intermolecular forces, the higher the boiling point.point.

• For compounds with approximately the same molecular weight:For compounds with approximately the same molecular weight:

Physical PropertiesPhysical Properties

Consider the example below. Note that the relative strength of the Consider the example below. Note that the relative strength of the intermolecular forces increases from pentane to butanal to 1-intermolecular forces increases from pentane to butanal to 1-butanol. The boiling points of these compounds increase in the butanol. The boiling points of these compounds increase in the same order.same order.

For two compounds with similar functional groups:For two compounds with similar functional groups:

• The larger the surface area, the higher the boiling point.The larger the surface area, the higher the boiling point.

• The more polarizable the atoms, the higher the boiling point.The more polarizable the atoms, the higher the boiling point.



Consider the examples below which illustrate the effect of size and Consider the examples below which illustrate the effect of size and polarizability on boiling points.polarizability on boiling points.



Liquids having different boiling points can be separated in the Liquids having different boiling points can be separated in the laboratory using a distillation apparatus, shown in Figure 3.4.laboratory using a distillation apparatus, shown in Figure 3.4.



Boiling Point and Melting PointBoiling Point and Melting Point

Physical PropertiesPhysical PropertiesFactors Affecting Boiling Point and Melting PointFactors Affecting Boiling Point and Melting Point1.1. Molecular WeightMolecular Weight

The higher the molecular weight, the higher the melting point The higher the molecular weight, the higher the melting point and boiling point.and boiling point.

a.a. The hydrocarbon chain gets longer, LDF gets strongerThe hydrocarbon chain gets longer, LDF gets strongerb.b. The molecules get denser and find it difficult to volatilize due The molecules get denser and find it difficult to volatilize due

to the effect of gravitational forceto the effect of gravitational force

2.2. BranchingBranchingThe more branched a compound is, the lower the boiling pointThe more branched a compound is, the lower the boiling pointa.a. The surface area for interaction gets shorterThe surface area for interaction gets shorterb.b. The molecule cannot get close enough for stronger The molecule cannot get close enough for stronger

interactioninteraction

Physical PropertiesPhysical PropertiesFactors Affecting Boiling Point and Melting PointFactors Affecting Boiling Point and Melting Point3.3. PolarityPolarity

The more polar the compound, the stronger the dipole-dipole The more polar the compound, the stronger the dipole-dipole interactions, the higher the boiling point and melting point.interactions, the higher the boiling point and melting point.

4.4. Molecular SymmetryMolecular SymmetrySymmetrical molecules have lower boiling point than their Symmetrical molecules have lower boiling point than their

asymmetrical counterpartsasymmetrical counterpartsThe dipole moment in symmetrical molecules cancel out so that The dipole moment in symmetrical molecules cancel out so that

as a whole the molecules become nonpolar and therefore as a whole the molecules become nonpolar and therefore tend to form weaker interaction.tend to form weaker interaction.

5.5. Intramolecular H-BondingIntramolecular H-BondingThe presence of intramolecular H-Bonding decreses boiling The presence of intramolecular H-Bonding decreses boiling point point or meting point.or meting point.

Boiling Point and Melting PointBoiling Point and Melting Point

Boiling PointBoiling Point

Compound Boiling Point Compound Boiling Point1. Alkanes 3. Alkynes

propane -42.07°C Ethyne -84

n-butane 0.50 Propyne -23.22

n-pentane 36.07 1-butyne 8.07

isopentane 27.85 1-pentyne 40.18

neopentane 9.50 1-hexyne 71.33

2. Alkenes 4. Aromatics

ethene -103.71 Benzene 80.09

propene -47.70 Phenol 182

1-butene -6.26 O-nitrophenol 214.5

1-pentene +29.97 P-nitrophenol 279.0

1-hexene +63.49 toluene 110.6

aniline 184.4

Boiling PointBoiling Point

Compound MW Boiling Point1. n-butyl amine 73 78°C

2. n-butyl alcohol 74 118

3. Diethyl ether 74 35

4. Butanal 72 76

5. Propanoic acid 74 141

6. Butanoic acid 88 164

7. n-pentyl alcohol 88 138

• The The melting point melting point is the temperature at which a solid is converted is the temperature at which a solid is converted to its liquid phase.to its liquid phase.

• In melting, energy is needed to overcome the attractive forces in In melting, energy is needed to overcome the attractive forces in the more ordered crystalline solid.the more ordered crystalline solid.

• The stronger the intermolecular forces, the higher the melting The stronger the intermolecular forces, the higher the melting point.point.

• Given the same functional group, the more symmetrical the Given the same functional group, the more symmetrical the compound, the higher the melting point.compound, the higher the melting point.


Physical Properties — Melting PointPhysical Properties — Melting Point

• Because ionic compounds are held together by extremely strong Because ionic compounds are held together by extremely strong interactions, they have very high melting points.interactions, they have very high melting points.

• With covalent molecules, the melting point depends upon the With covalent molecules, the melting point depends upon the identity of the functional group. For compounds of approximately identity of the functional group. For compounds of approximately the same molecular weight:the same molecular weight:



• The trend in melting points of pentane, butanal, and 1-butanol The trend in melting points of pentane, butanal, and 1-butanol parallels the trend observed in their boiling points.parallels the trend observed in their boiling points.



• Symmetry also plays a role in determining the melting points of Symmetry also plays a role in determining the melting points of compounds having the same functional group and similar compounds having the same functional group and similar molecular weights, but very different shapes.molecular weights, but very different shapes.

• A compact symmetrical molecule like neopentane packs well into a A compact symmetrical molecule like neopentane packs well into a crystalline lattice whereas isopentane, which has a CHcrystalline lattice whereas isopentane, which has a CH33 group group

dangling from a four-carbon chain, does not. Thus, neopentane dangling from a four-carbon chain, does not. Thus, neopentane has a much higher melting point.has a much higher melting point.



Melting PointMelting Point

Compound Melting Point (K)1. o-hydroxybenzaldehyde 2662. p-hydroxybenzaldehyde 388

3. o-methoxybenzaldehyde 3094. p-methoxybenzaldehyde 273

5. o-aminobenzaldehyde 3126. p-aminobenzaldehyde 344

• SolubilitySolubility is the extent to which a compound, called a solute, is the extent to which a compound, called a solute, dissolves in a liquid, called a solvent. dissolves in a liquid, called a solvent.

• In dissolving a In dissolving a compound, the energy compound, the energy needed to break up the needed to break up the interactions between interactions between the molecules or ions the molecules or ions of the solute comes of the solute comes from new interactions from new interactions between the solute and between the solute and the solvent.the solvent.


Physical Properties — SolubilityPhysical Properties — Solubility

• Compounds dissolve in solvents having similar kinds of Compounds dissolve in solvents having similar kinds of intermolecular forces.intermolecular forces.

• ““Like dissolves like.”Like dissolves like.”

• Polar compounds dissolve in polar solvents. Nonpolar or weakly Polar compounds dissolve in polar solvents. Nonpolar or weakly polar compounds dissolve in nonpolar or weakly polar solvents.polar compounds dissolve in nonpolar or weakly polar solvents.

• Water and organic solvents are two different kinds of solvents. Water and organic solvents are two different kinds of solvents. Water is very polar since it is capable of hydrogen bonding with a Water is very polar since it is capable of hydrogen bonding with a solute. Many organic solvents are either nonpolar, like carbon solute. Many organic solvents are either nonpolar, like carbon tetrachloride (CCltetrachloride (CCl44) and hexane [CH) and hexane [CH33(CH(CH22))44CHCH33], or weakly polar, ], or weakly polar,

like diethyl ether (CHlike diethyl ether (CH33CHCH22OCHOCH22CHCH33).).

• Most ionic compounds are soluble in water, but insoluble in organic Most ionic compounds are soluble in water, but insoluble in organic solvents.solvents.



• An organic compound is water soluble only if it contains one polar An organic compound is water soluble only if it contains one polar functional group capable of hydrogen bonding with the solvent for functional group capable of hydrogen bonding with the solvent for every five C atoms it contains. For example, compare the solubility every five C atoms it contains. For example, compare the solubility of butane and acetone in Hof butane and acetone in H22O and CClO and CCl44..



• Since butane and acetone are both organic compounds having a CSince butane and acetone are both organic compounds having a C—C and C—H backbone, they are soluble in the organic solvent —C and C—H backbone, they are soluble in the organic solvent CClCCl44. Butane, which is nonpolar, is insoluble in H. Butane, which is nonpolar, is insoluble in H22O. Acetone is O. Acetone is

soluble in Hsoluble in H22O because it contains only three C atoms and its O O because it contains only three C atoms and its O

atom can hydrogen bond with an H atom of Hatom can hydrogen bond with an H atom of H22O.O.



• To dissolve an ionic compound, the strong ion-ion interactions To dissolve an ionic compound, the strong ion-ion interactions must be replaced by many weaker ion-dipole interactions.must be replaced by many weaker ion-dipole interactions.



• The size of an organic molecule with a polar functional group The size of an organic molecule with a polar functional group determines its water solubility. A low molecular weight alcohol like determines its water solubility. A low molecular weight alcohol like ethanol is water soluble since it has a small carbon skeleton of ethanol is water soluble since it has a small carbon skeleton of five C atoms), compared to the size of its polar OH group. five C atoms), compared to the size of its polar OH group. Cholesterol has 27 carbon atoms and only one OH group. Its Cholesterol has 27 carbon atoms and only one OH group. Its carbon skeleton is too large for the OH group to solubilize by carbon skeleton is too large for the OH group to solubilize by hydrogen bonding, so cholesterol is insoluble in water.hydrogen bonding, so cholesterol is insoluble in water.



• The nonpolar part of a molecule that is not attracted to HThe nonpolar part of a molecule that is not attracted to H22O is said O is said

to be to be hydrophobichydrophobic..

• The polar part of a molecule that can hydrogen bond to HThe polar part of a molecule that can hydrogen bond to H22O is said O is said

to be to be hydrophilichydrophilic..

• In cholesterol, for example, the hydroxy group is hydrophilic, In cholesterol, for example, the hydroxy group is hydrophilic, whereas the carbon skeleton is hydrophobic.whereas the carbon skeleton is hydrophobic.



Factors Affecting Solubility in WaterFactors Affecting Solubility in Water1.1. Nature of the SoluteNature of the Solute

RCOOH, ROH, RNHRCOOH, ROH, RNH22 > RCHO, RCOR > ROR > HC > RCHO, RCOR > ROR > HC- with reciprocal H-bonding - with reciprocal H-bonding

2.2. Carbon- Chain lengthCarbon- Chain lengthWithin a functional group, solubility in water decrease as Within a functional group, solubility in water decrease as

molecular weight increasesmolecular weight increasesa.a. As MW ↑ , the carbon chain gets longerAs MW ↑ , the carbon chain gets longerb.b. That portion being hydrophobic, solubility in water decreases That portion being hydrophobic, solubility in water decreases

and the compound instead becomes soluble in nonpolar and the compound instead becomes soluble in nonpolar solvents.solvents.



Factors Affecting Solubility in WaterFactors Affecting Solubility in Water3.3. BranchingBranching

As the molecule becomes more branched, the solubility in water As the molecule becomes more branched, the solubility in water increases.increases.

As the extent of branching increases, the hydorphobic portion As the extent of branching increases, the hydorphobic portion gets shorter and that favors the formation of H-bond with gets shorter and that favors the formation of H-bond with water.water.

4.4. Tendency to form intramolecular H-bondTendency to form intramolecular H-bondThis decreases the tendency of the compound to form H-bond This decreases the tendency of the compound to form H-bond

with water thus decreases its solubility in the solvent. with water thus decreases its solubility in the solvent.



1.1. Alcohol Alcohol four-carbon dividing line four-carbon dividing line2.2. AminesAmines 1° > 2° > 3° 1° > 2° > 3°3.3. Alkyl halidesAlkyl halides4.4. EthersEthers



Vitamins are either lipid or water soluble.Vitamins are either lipid or water soluble.


Application - VitaminsApplication - Vitamins

Soap:Soap:

Soap molecules have Soap molecules have two distinct parts—a two distinct parts—a hydrophilic portion hydrophilic portion composed of ions composed of ions called the polar head, called the polar head, and a hydrophobic and a hydrophobic carbon chain of carbon chain of nonpolar C—C and Cnonpolar C—C and C—H bonds, called the —H bonds, called the nonpolar tail.nonpolar tail.


Application - SoapApplication - Soap


Application – The cell membraneApplication – The cell membrane

Transport Across a Cell Membrane: Transport Across a Cell Membrane: • Polar molecules and ions are transported across cell membranes Polar molecules and ions are transported across cell membranes

encapsulated within molecules called ionophores.encapsulated within molecules called ionophores.• IonophoresIonophores are organic molecules that complex cations. They have a are organic molecules that complex cations. They have a

hydrophobic exterior that makes them soluble in the nonpolar interior of the hydrophobic exterior that makes them soluble in the nonpolar interior of the cell membrane, and a central cavity with several oxygens whose lone pairs cell membrane, and a central cavity with several oxygens whose lone pairs complex with a given ion.complex with a given ion.



Transport Across a Cell Membrane: Transport Across a Cell Membrane:



Several synthetic ionophores have also been prepared, including one Several synthetic ionophores have also been prepared, including one group called crown ethers. group called crown ethers.

Crown ethers are cyclic ethers containing several oxygen atoms that Crown ethers are cyclic ethers containing several oxygen atoms that bind specific cations depending on the size of their cavity.bind specific cations depending on the size of their cavity.



Rank the following groups of compounds in order of Rank the following groups of compounds in order of decreasing solubility in water:decreasing solubility in water:

Physical Properties - SolubilityPhysical Properties - Solubility

Exercise:Exercise:

Criteria for AromaticityCriteria for Aromaticity

AromaticityAromaticity

Aromatic compounds are particularly stable.Aromatic compounds are particularly stable.


To be classified as aromatic, a compound must meet To be classified as aromatic, a compound must meet both of the following criteria:both of the following criteria:

1. It must have an uninterrupted cyclic cloud of 1. It must have an uninterrupted cyclic cloud of ππ electrons electrons (often called a (often called a ππ cloud) above and below the plane of the cloud) above and below the plane of the molecule. Let’s look a little more closely molecule. Let’s look a little more closely at what this means:at what this means:

For the For the ππ cloud to be cyclic, cloud to be cyclic, the molecule must be cyclic.the molecule must be cyclic.For the For the ππ cloud to be uninterrupted, cloud to be uninterrupted, every atom in the ring every atom in the ring

must have a p must have a p orbital.orbital.For the For the ππ cloud to form, cloud to form, each p orbital must overlap with the each p orbital must overlap with the

p orbitals on either side of itp orbitals on either side of it. . Therefore, the molecule Therefore, the molecule must be planar.must be planar.

2. The 2. The ππ cloud must contain an odd number of pairs of cloud must contain an odd number of pairs of ππ electrons.electrons.


Hückel’s rule, or the 4n +2 ruleHückel’s rule, or the 4n +2 rule for a planar, cyclic compound to be aromatic, its for a planar, cyclic compound to be aromatic, its

uninterrupted uninterrupted ππ cloud must contain (4n+2) cloud must contain (4n+2) π π electrons, electrons, where where n is any whole number.n is any whole number.

For a compound to be aromatic, it must be cyclic and For a compound to be aromatic, it must be cyclic and planar and have an uninterrupted pi cloud of electrons. planar and have an uninterrupted pi cloud of electrons. The cloud must contain an odd number of The cloud must contain an odd number of pairs of pi pairs of pi electrons.electrons.

Example:Example:a. What is the value of n in Hückel’s rule when a compound a. What is the value of n in Hückel’s rule when a compound

has nine pairs of has nine pairs of ππ electrons? electrons?b. Is such a compound aromatic?b. Is such a compound aromatic?

AnnulenesAnnulenesMonocyclic hydrocarbons with alternating single and Monocyclic hydrocarbons with alternating single and

double bondsdouble bondsA prefix in brackets denotes the number of carbons in A prefix in brackets denotes the number of carbons in

the ring.the ring.


Aromatic?Aromatic?

Which of the following compounds are aromatic?Which of the following compounds are aromatic?


Exercise:Exercise:

Aromatic Heterocyclic CompoundsAromatic Heterocyclic Compounds


A A heterocyclic compound heterocyclic compound is a cyclic compound in which one is a cyclic compound in which one or more of the ring atoms is an atom other than carbon.or more of the ring atoms is an atom other than carbon.

AntiaromaticityAntiaromaticity


Antiaromatic compounds are highly unstable.Antiaromatic compounds are highly unstable.

AromaticityAromaticity is characterized by stability, whereas is characterized by stability, whereas antiaromaticityantiaromaticity is characterized by instability. is characterized by instability.

it must be a planar, cyclic compound with an uninterrupted it must be a planar, cyclic compound with an uninterrupted ring of p orbital-bearing atoms, and the ring of p orbital-bearing atoms, and the ππ cloud must contain cloud must contain an even number of pairs of an even number of pairs of π π electrons.electrons.

Antiaromatic CompoundsAntiaromatic Compounds


it must be a planar, cyclic compound with an uninterrupted it must be a planar, cyclic compound with an uninterrupted ring of p orbital-bearing atoms, and the ring of p orbital-bearing atoms, and the ππ cloud must contain cloud must contain an even number of pairs of an even number of pairs of π π electrons.electrons.

Antiaromatic CompoundsAntiaromatic Compounds


Aromatic compounds are stable Aromatic compounds are stable because they have filled because they have filled bonding bonding ππ molecular orbitals.molecular orbitals.

structural effects

Documents

strong electronegative

intermolecular

factors affecting

pi electron

inductive

electron delocalization

repelling

intermolecular