general, organic, and biological chemistry: an integrated approach laura frost, todd deal and karen...

General, Organic, and Biological Chemistry: An Integrated Approach

Laura Frost, Todd Deal and Karen Timberlakeby Richard Triplett

INTERMOLECULAR FORCES:STATE CHANGES, SOLUBILITY,

AND CELL MEMBRANES

CHAPTER 6

CHAPTER OUTLINE

6.1 Types of Intermolecular Forces

6.2 Intermolecular Forces and Solubility

6.3 Intermolecular Forces and Changes of State

6.4 Fats, Oils, and Margarine—Solid to Liquid and

Back Again: Melting

6.5 Intermolecular Forces and Cell Membrane

© 2011 Pearson Education, Inc.

Chapter 6 2

OBJECTIVES FOR TODAY

Examine types of intermolecular forces

Investigate the relationship between intermolecular forces and solubility


Chapter 6 3

WHAT ARE INTERMOLECULAR FORCES?

Intermolecular forces are attractive forces between molecules that occur when there is a variation in the electron distribution in a molecule.

Intermolecular forces are weaker than the weakest covalent bonds (intramolecular).

Intermolecular forces arise when a partially negative charge on a molecule is attracted to a partially positive charge on another molecule.© 2011 Pearson Education, Inc.

Chapter 6 4

TYPES OF INTERMOLECULAR FORCES

5

London Forces

London forces occur momentarily in all molecules when electrons become unevenly distributed over a molecule’s surface.

A temporary unequal charge, or induced dipole, is momentarily created in a molecule that attracts the electrons of a second molecule, creating an attraction between these two molecules. © 2011 Pearson Education, Inc.

Chapter 6 6

London forces are significant only in nonpolar molecules because they are the only intermolecular force occurring in nonpolar molecules.

Plastic wraps are made of long chain, nonpolar hydrocarbons. When plastic wrap folds back on itself, these long chains interact with each other creating a temporary dipole. This temporary dipole causes the wrap to stick to itself.


Chapter 6 7


Chapter 6 8

Dipole–Dipole Attractions

Dipole–dipole attractions occur between the dipoles of two polar molecules and are caused by the permanent, uneven distribution of electrons, which is caused by the electronegativity differences of atoms in the molecule.

These attractions are stronger than London forces because the dipoles of polar molecules is permanent.

9


Chapter 6 10

Dipole–dipole attractions do not exist between nonpolar molecules.

Polar molecules also have London forces, but the attraction of the dipoles is much stronger, making the London forces negligible.


Chapter 6 11

Hydrogen Bonding

Some molecules have a strong attraction to each other due to a large dipole that arises from the partial charges on particular atoms.

This force is known as hydrogen bonding. It is a very strong dipole–dipole attraction between two molecules and involves hydrogen.


Chapter 6 12

Hydrogen bonding requires the interaction between two molecules, a donor and an acceptor.

Requirements for hydrogen bonding are shown in the following table.


Chapter 6 13

The high electronegativity of O, N, and F polarizes the hydrogen atom of the donor with a partial positive charge.

This partial positive charge strongly attracts the high partial negative charge on the nonbonding electron pair of O, N, or F on the acceptor.

This type of polarization and attraction, shown on the next slide, occurs between water molecules.


Chapter 6 14


Chapter 6 15

Hydrogen Bonding in Water

Hydrogen bonding occurs between the same molecules as seen in water, between two different polar molecules, or even between different parts of the same molecule.


Chapter 6 16

Ion–Dipole Attraction: A Similar Attractive Force

• An ion-dipole attraction is an attraction of an ion to the opposite partial charge on a polar molecule.

• This type of attraction occurs when table salt is dissolved in water. The sodium ion is attracted to the partial negative charge of water, and the chloride ion is attracted to the partial positive charge of water.


Chapter 6 17

An ion–dipole attraction plays an important role in the solubility of ionic compounds.


Chapter 6 18

INTERMOLECULAR FORCES AND SOLUBILITY

19Chapter 6

The Golden Rule of Solubility

The golden rule of solubility—like dissolves like—means that molecules that are similar will dissolve each other.

Molecules that have similar polarity and participate in the same types of intermolecular forces will dissolve each other.


Chapter 6 20

Applying the Golden Rule to Nonpolar

Compounds

Oil and water do not mix.

Dietary oils, known as triglycerides, are nonpolar organic compounds formed through the condensation reaction of glycerol with three fatty acids. This reaction is called esterification because an ester functional group is formed. © 2011 Pearson Education, Inc.

Chapter 6 21


Chapter 6 22

Oils are nonpolar and attracted to neighboring molecules through London forces. Since water is polar, oil and water do not interact with each other, and therefore, they do not dissolve in each other.

The attraction forces between water molecules are much greater than the attraction forces between a water molecule and an oil molecule.

If a bottle of oil and vinegar salad dressing are mixed the contents will separate upon standing.


Chapter 6 23

Applying the Golden Rule to Polar Compounds

When table sugar is added to water, it will dissolve. The hydroxyl groups on the sugar molecule make it a polar molecule.

The hydroxyl groups of sugar interact with water through hydrogen bonding. Dipole–dipole interactions also occur.


Chapter 6 24


Chapter 6 25

Applying the Golden Rule to Ionic Compounds

Ion–dipole attractions can exist between water and ions.

The intermolecular attractions between water and ionic compounds are stronger than the attractions between water and polar covalent compounds. This strong attraction often makes ionic compounds more soluble in water than covalent compounds. © 2011 Pearson Education, Inc.

Chapter 6 26

The ion–dipole attractions between ions of an ionic compound and water are so strong that the ionic bond between ions is disrupted.

When multiple water molecules interact with an ion, the sum of these attractive forces is greater than the strength of the ionic bonds.

When an ionic compound, such as sodium chloride, interacts with water, a process known as hydration occurs, during which ions are surrounded by water molecules.© 2011 Pearson Education, Inc.

Chapter 6 27


Chapter 6 28

The Unique Chemistry of Soap

Soap molecules undergo intermolecular attractive forces.

Soaps are composed of fatty acid salts, which are ionic compounds. The carboxylic acid functional group in fatty acid salts are ionic because they contain the carboxylate form of the functional group. © 2011 Pearson Education, Inc.

Chapter 6 29

The charge on the carboxylate group makes this end of the molecule ionic and polar. The remaining part of the molecule is nonpolar.

Molecules like soap have a polar and nonpolar end and are known as amphipathic compounds.


Chapter 6 30

Amphipathic compounds do not dissolve in water. The nonpolar tails are hydrophobic (water fearing) and are excluded from water.


Chapter 6 31

The ionic head of a soap molecule is hydrophilic (water loving) and interacts with water through ion–dipole interactions.

Nonpolar hydrocarbon tails of soap interact with each other and form a core of a spherical molecule known as a micelle. Water is excluded from the inner core.

The polar heads interact with water and form the outer shell of this micelle. © 2011 Pearson Education, Inc.

Chapter 6 32

Soap works by trapping grease and dirt, which are nonpolar, in the inner core of the micelle, which then washes it away with water surrounding the outer shell. © 2011 Pearson Education, Inc.

Chapter 6 33

OBJECTIVES FOR TODAY

Examine types of intermolecular forces

Investigate the relationship between intermolecular forces and solubility


Chapter 6 34

OBJECTIVE FOR TODAY

Examine the relationships between intermolecular forces and changes of phase


Chapter 6 35

6.3 INTERMOLECULAR FORCES AND CHANGES OF STATE

Heat and Intermolecular Forces

Intermolecular forces between two molecules are stronger when they are moving slowly than when they are moving more rapidly.

When a substance is heated, its molecules move more rapidly, resulting in a decrease in the intermolecular forces between molecules.


Chapter 6 36

6.3 INTERMOLECULAR FORCES AND CHANGES OF STATE, CONTINUED

When a solid is heated, it melts to form a liquid, and when a liquid is heated, it evaporates to form a gas.

These transitions are called changes of state or phase transitions.

The application of heat to a substance causes its molecules to move faster, which disrupts the intermolecular forces holding the molecules together and causes a change from one state to another. © 2011 Pearson Education, Inc.

Chapter 6 37


Boiling Points and Alkanes

Why do compounds that are similar boil at different temperatures?

First, we need to understand what happens at the molecular level during boiling.


Chapter 6 38


The temperature at which all molecules of a substance change from a liquid to a gas is called the boiling point.

Two things must happen during the boiling process:

1. Molecules of the substance must push back the molecules of the atmosphere at the liquid surface.

2. Molecules must overcome the attractive forces so they can move into the gas phase.


Chapter 6 39


Heat supplied during boiling provides the energy necessary to overcome these two tasks.

The molecules of the atmosphere stay the same no matter what the liquid surface is, so the difference between boiling points must be due to the intermolecular forces.© 2011 Pearson Education, Inc.

Chapter 6 40


Consider the two alkanes, pentane and octane. Pentane boils at 36 oC and octane boils at 125 oC.

Pentane is a five-carbon, straight-chain alkane, whereas octane is an eight-carbon, straight-chain alkane.

Both are nonpolar molecules that only exhibit London forces. © 2011 Pearson Education, Inc.

Chapter 6 41


Because octane is a larger molecule than pentane, there is more surface area for molecular interaction.

The larger surface area means that stronger London forces are exhibited by octane molecules than the forces seen in a smaller molecule like pentane.

The stronger attraction forces between molecules must be overcome in order for the compound to boil. © 2011 Pearson Education, Inc.

Chapter 6 42


More heat is necessary to disrupt these attractions, which means the boiling point is higher for octane than for pentane.


Chapter 6 43


This table shows the boiling point of some common straight-chain alkanes. As the number of carbon atoms increases, the boiling point increases.


Chapter 6 44


Consider the alkanes, hexane and 2,3-dimethylbutane, shown in this figure.


Chapter 6 45


These two compounds are structural isomers and have the same number of carbon atoms.

Hexane is a straight-chain alkane and 2,3-dimethylbutane is a branched-chain alkane.

Hexane has a boiling point of 69 oC and 2,3-dimethylbutane has a boiling point of 58 oC.

This difference is due to the differences in surface area of the two molecules. © 2011 Pearson Education, Inc.

Chapter 6 46


Hexane has a larger surface area, which results in stronger London forces than those occurring in 2,3-dimethylbutane.

More energy (heat) is required to overcome these stronger attractions in hexane, resulting in a higher boiling point, when compared to the boiling point of 2,3-dimethylbutane.

For alkanes with the same number of carbon atoms, straight-chain alkanes have higher boiling points than branched-chain alkanes. © 2011 Pearson Education, Inc.

Chapter 6 47



Chapter 6 48


The Unusual Behavior of Water

Water is a small molecule with only three atoms. Compare it to propane, which has 11 atoms.

In terms of surface area, one might think that water would have a lower boiling point than propane. However, water boils at 100 oC, and propane boils at -42 oC. Something other than surface area accounts for this difference.


Chapter 6 49


Water molecules are held together through hydrogen bonding, which is a much stronger force than the London forces observed in propane.

More heat is required to disrupt the hydrogen bond interaction in water.

More heat required means higher boiling point.


Chapter 6 50

6.4 FATS, OILS, AND MARGARINE—SOLID TO LIQUIDAND BACK AGAIN: MELTING

Fats

Fat, derived from animals, is a solid or semi-solid at room temperature, whereas oil is a liquid at room temperature.

Fats, like oils, are triglycerides made up of three fatty acid molecules joined to a glycerol backbone.

Intermolecular attractions explain why fats are solid and oils are liquids at room temperature. © 2011 Pearson Education, Inc.

Chapter 6 51

6.4 FATS, OILS, AND MARGARINE—SOLID TO LIQUIDAND BACK AGAIN: MELTING, CONTINUED

When the hydrocarbon chains of fatty acids are mostly saturated, the triglyceride is a fat. If the chains are mostly unsaturated, the triglyceride is an oil.

The saturated hydrocarbon chains allow the fatty acid chains in a fat to be closer together, allowing for more London forces.

The unsaturated hydrocarbon chains of an oil are farther apart, which results in a decrease in attractive forces between the chains. © 2011 Pearson Education, Inc.

Chapter 6 52



Chapter 6 53


The increase in molecular interactions in a fat slows the motion of the molecules down, thereby allowing the molecules of a fat to form a solid.

Fats melt at body temperature because the intermolecular forces that must be disrupted are weak London forces.

Because the melting points are low, fats are often referred to as semisolids.


Chapter 6 54


Oils

Oils are derived from plants and are liquids at room temperature.

The hydrocarbon chains of oils are unsaturated, containing double bonds with a cis configuration.

The cis configuration forms kinks in the hydrocarbon chains, preventing chains from stacking together as closely as those in a fat.


Chapter 6 55


This decrease in interaction results in less London forces, which means the chains in oils are less attracted to one another and move more freely.

Greater molecular movement of the hydrocarbon tails in an oil does not allow enough stacking of the tails to form a solid, therefore oils are liquids at room temperature.


Chapter 6 56



Chapter 6 57


Oils derived from plants are healthier alternatives for human dietary fat requirements.

Oils are not always the most convenient form of triglycerides.

Chemists have developed methods to convert liquid plant oils into solids like margarine.

Conversion of liquid oils to margarine requires a process known as hydrogenation.© 2011 Pearson Education, Inc.

Chapter 6 58


Hydrogenation involves the addition of hydrogen atoms to the carbon–carbon double bond of an unsaturated compound.

Hydrogenation progresses more rapidly if a catalyst is included in the reaction. A catalyst speeds up a reaction but remains unchanged at the completion of the reaction.


Chapter 6 59


Trans Fats

Margarine is more spreadable than cold butter because cold butter is a firmer solid than cold margarine.

Margarine is more spreadable because the hydrogenation of plant oils is controlled, such that some of the double bonds become saturated, while others remain intact.


Chapter 6 60


This controlled process, known as partial hydrogenation, allows production of margarines that are solid yet easier to spread than more solid butter.

Partial hydrogenation produces margarines that are less saturated than butter, making them easier to spread.

A consequence of partial hydrogenation is that some of the double bonds are incompletely hydrogenated. © 2011 Pearson Education, Inc.

Chapter 6 61


Incomplete hydrogenation of double bonds causes the favorable cis configuration of the double bonds to convert to the less favorable trans configuration, resulting in the compounds known as trans fats.

Some studies have shown that trans fats have deleterious health effects. This led to consideration of alternatives. Food labels must contain the amounts of trans fatty acids present in food.© 2011 Pearson Education, Inc.

Chapter 6 62



Chapter 6 63

OBJECTIVE FOR TODAY

Examine the relationships between intermolecular forces and changes of phase


Chapter 6 64

OBJECTIVE FOR TODAY

Examine the relationships between intermolecular forces and cell membranes


Chapter 6 65

6.5 INTERMOLECULAR FORCES AND THECELL MEMBRANE

A Quick Look at Phospholipids

Phospholipids are the primary structural components of cell membranes.

Phospholipids have a glycerol backbone with two fatty acids linked to glycerol through an ester bond. The third OH group of glycerol is linked to a phosphate-containing group instead of a fatty acid.


Chapter 6 66

6.5 INTERMOLECULAR FORCES AND THECELL MEMBRANE, CONTINUED

The phosphate-containing group is ionic (polar), which is in contrast to the fatty acid tails (nonpolar).

Because phospholipids have both a polar and nonpolar part, they are amphipathic.

The fatty acid tails and phosphate-containing group are arranged as shown on the next slide.


Chapter 6 67



Chapter 6 68


Having two nonpolar tails and a large polar head enhances the nonpolar and polar characteristics.

The two tails of phospholipids affect the overall shape of the molecule.

Phospholipids tend to have a cylindrical shape with a large polar head group. This shape hinders their ability to form micelles like soap molecules.


Chapter 6 69


The Cell Membrane Is a Bilayer

With an aqueous environment inside and outside the cell, the cell membrane, composed of phospholipids, cannot have a single layer of phospholipids.

The phospholipids form a double layer called a bilayer. The polar heads of the phospholipids on the outside and inside of the cell are oriented so that the polar heads are directed to the aqueous environments. © 2011 Pearson Education, Inc.

Chapter 6 70


The nonpolar tails are oriented toward each other, creating a nonpolar interior region.

The phospholipid bilayer is the foundation of the cell membrane.© 2011 Pearson Education, Inc.

Chapter 6 71


Proteins are one of the most important components in the phospholipid bilayer of the cell membrane.

Protein molecules can span the lipid bilayer, extending outward on either side (integral membrane proteins) or they can associate with one polar surface of the cell membrane (peripheral membrane proteins).

Proteins aid in allowing molecules to move in and out of the cell.© 2011 Pearson Education, Inc.

Chapter 6 72


The current model for the cell membrane is called the fluid mosaic model.

Proteins are free to move about in this sea of lipids, implying that the membrane is fluid-like because the proteins are able to move freely in the bilayer.

Cholesterol is also embedded in the lipid bilayer.


Chapter 6 73



Chapter 6 74


Cholesterol in Membranes

The cholesterol molecule has a polar and a nonpolar portion.

The cholesterol molecule is embedded within the lipid bilayer, so that the polar end protrudes out into the surrounding aqueous environment.

The nonpolar portion is contained within the nonpolar interior of the membrane.© 2011 Pearson Education, Inc.

Chapter 6 75


Cholesterol modulates the fluidity or flexibility of the cell membrane. It can slip in between the phospholipids’ tails and can disrupt the London forces, which increases the fluidity of the membrane. It can also increase the rigidity of the membrane.© 2011 Pearson Education, Inc.

Chapter 6 76

OBJECTIVE FOR TODAY

Examine the relationships between intermolecular forces and cell membranes


Chapter 6 77

CHAPTER SUMMARY

6.1 Types of Intermolecular Forces

Intermolecular forces are weaker than covalent bonds.

There are three main types of intermolecular forces:

1. London forces. These are the weakest forces.

2. Dipole–dipole attractions

3. Hydrogen bonding. These are the strongest forces.


Chapter 6 78

CHAPTER SUMMARY, CONTINUED

6.1 Types of Intermolecular Forces, Continued

Each of the intermolecular forces involves the attraction of a partially negative charge on a molecule to a partially positive charge on a second molecule.


Chapter 6 79


6.2 Intermolecular Forces and Solubility

The rule of solubility is like dissolves like.

This rule means that polar molecules dissolve other polar molecules, and nonpolar molecules dissolve other nonpolar molecules.

Soap molecules dissolve nonpolar molecules by incorporating these molecules in spherical structures called micelles.© 2011 Pearson Education, Inc.

Chapter 6 80


6.3 Intermolecular Forces and Changes of State

The boiling point is the temperature at which the state of change from liquid to gas occurs.

Molecules that form strong intermolecular forces require more heat energy to disrupt these forces and have higher boiling points.


Chapter 6 81


6.3 Intermolecular Forces and Changes of

State, Continued

Molecules like alkanes have only London forces, so boiling points of these molecules depend on the surface area of the molecule. The greater the surface area, the higher the boiling point.

The more branched an alkane, the lower the boiling point, when compared to an alkane with the same number of carbon atoms.


Chapter 6 82


6.4 Fats, Oils, and Margarine—Solid to Liquid

and Back Again: Melting

Melting is the change of state from solid to liquid.

Fats, from animals, are solid triglycerides with low melting points because their molecules are held together with London forces.

Oils, from plants, are liquid at room temperature.© 2011 Pearson Education, Inc.

Chapter 6 83



and Back Again: Melting, Continued

The fatty acid tails of oils are highly unsaturated and the configuration about the double bond is cis. This cis configuration forms a kink in the molecule so the fatty acid tails are not able to attract each other to form London forces.

Oils can be converted into fats by hydrogenation, which is a process used to prepare margarine.© 2011 Pearson Education, Inc.

Chapter 6 84



and Back Again: Melting, Continued

Hydrogenation is difficult to control and results in converting the cis configuration of double bonds to the trans configuration.

The resulting fatty acids are called trans fats and are less healthy.


Chapter 6 85


6.5 Intermolecular Forces and the Cell

Membrane

Phospholipids are the major lipids in cell membranes. The cell membrane is a bilayer of phospholipids, with the polar portion of the phospholipids exposed to the aqueous environment outside and inside the cell.

The fluid mosaic model describes the structure of the cell membrane.© 2011 Pearson Education, Inc.

Chapter 6 86


6.5 Intermolecular Forces and the Cell

Membrane, Continued

Proteins are embedded within the cell membrane.

Cholesterol is also a part of the cell membrane and helps in the fluidity of the membrane.


Chapter 6 87

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