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3. Organic Compounds:Alkanes and Cycloalkanes

Based onMcMurry’s Organic Chemistry, 8th edition, Chapter 3

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Families of Organic Compounds

! Structural features that make it possible to classifycompounds by reactivity are call functional groups

! Organic compounds can be grouped into families by theircommon structural features

! Chapter 3 deals with structure of some common functionalgroups but emphasizes alkanes and alkane isomers

! Alkanes are compounds that contain only carbons andhydrogens, all connected exclusively by single bonds

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3.1 Functional Groups

! A functional group is a collection of atoms at a sitewithin a molecule with a common bonding pattern

! The group reacts in a typical way, generallyindependent of the rest of the molecule

! For example, a double bond in simple and/or complexalkene reacts with bromine in the same way; see below

BrBr

CH3 CH3BrBr

CH3 CH3

starting material

Br2

starting material

Br2

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Survey of Functional Groups

! Know Table 3.1 which lists a wide variety offunctional groups (also inside back cover)

! As you learn about them in each chapter andin laboratory (i.e., the FTIR experiment,page63), it will be easier to recognize them

! The functional groups affect the reactions,structure, and physical properties of everycompound in which they occur

! Know your MANTRA when I say, “reaction”

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Types of Functional Groups: MultipleCarbon–Carbon Bonds! *Alkenes have a

C=C double bond! *Alkynes have a

C÷C triple bond! *Arenes have

special bonds thatare represented asalternating singleand double C-Cbonds in a six-membered ring

! The arene shown isconjugated ! - " - !

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Functional Groups with Carbon Singly Bondedto an Electronegative Atom

! *Alkyl halide: C bonded to halogen (C-X)

! * Alcohol: C bonded O of a hydroxyl group (C-OH)

! * Ether: Two C’s bonded to the same O (C-O-C)

! * Amine: C bonded to N (C-NR2; where R=H or C)

! Thiol: C bonded to SH group (C-SH)

! Sulfide: Two C’s bonded to same S (C-S-C)! Bonds are polar, with partial positive charge on C (#+)

and partial negative charge (#$) on electronegativeatom; remember F O N Cl Br I S C H

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Groups with a Carbon–Oxygen Double Bond

(Carbonyl “C=O” derivatives)

! *Ketone: two C’s bonded to the carbonyl, C=O! *Aldehyde: one hydrogen bonded to C=O! *Carboxylic acid: %OH bonded to the C=O! *Ester: C-O bonded to the C=O, watch the

video, “additional words about the esters”! Amide: C-N bonded to the C=O! Acid halide: X bonded to the C=O! Carbonyl C has partial positive charge (#+)! Carbonyl O has partial negative charge (#-).

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amide

ester

ester

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3.2 Alkanes and Alkane Isomers

! Alkanes: Compounds with C-C single bonds and C-Hbonds only (no functional groups)

! Connecting carbons can lead to large or small molecules! The formula for an alkane with no rings in it must be

CnH2n+2 where the number of C’s is n! Alkanes are saturated with hydrogen (no more can be

added! Catentation -the formation of networks (often “chains”) of

identical atoms bond together

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Names of Small Hydrocarbons

C10H22decane10C9H20nonane9C8H18octane8C7H16heptane7C6H14hexane6C5H12pentane5C4H10butane4C3H8propane3C2H6ethane2CH4methane 1

(CnH2n+2)Formula NameNo. of Carbons

7535189532111

Possible alkane isomers“catenation”

C20H42isocane20 366,319

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Constitutional Isomers

! Same molecular formula, different structure to beisomers

! Isomers that differ in how their atoms are arranged inchains are called constitutional isomers

! Compounds other than alkanes can beconstitutional isomers of one another

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Alkane “Constitutional” Isomers

! Constitutional Isomers - Same formula differentstructure; C5H12

! straight-chain (continuous-chain) or normalalkanes

! branched-chain alkanes

!

2,2-dimethylpropane2-methylbutanen-pentane

2,2-dimethylpropane2-methylbutanen-pentane

2,2-dimethylpropane2-methylbutanen-pentane

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Condensed Structures of Alkanes

! A condensed structure does not show bondsbut lists atoms, such as isomers of pentane! CH3(CH2) 3CH3 (n-pentane)! (CH3) 2CH2CH2 CH3 (2-methylbutane)! CH3C(CH3)2CH3 (2,2-dimethylpropane)

2,2-dimethylpropane2-methylbutanen-pentane

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3.3 Alkyl Groups

! Alkyl group – remove one H from an alkane(a part of a structure)

! General abbreviation “R” (the “rest” of themolecule and should be defined, for example,R=H, CH3)

! Name: replace -ane ending of alkane with -ylending! _CH3 is “methyl” (from methane, Me-)

_CH2CH3 is “ethyl” (from ethane. Et-)

! See Table 3.4 for a summary of the above

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You must know your abbreviations:Me, methyl groupEt, ethyl groupn-Pr, i-Pr for the two propyl groups•n-Pr, normal straight chain propyl•i-Pr, isopropyl (branched)Four butyl groups (common names)n-Bu, sec-Bu, i-Bu, t-Bun-butyl, sec-butyl, isobutyl, t-butyl

Abbreviations

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Special positions (handed out in class)

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Types of Bond Positons

! Know your Bond Positions! a carbon at the end of a chain (primary alkyl group)

! a carbon in the middle of a chain (secondary alkylgroup)

! a carbon with three carbons attached to it (tertiary alkylgroup)

1° 2° 3° 4°

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! Draw all isomers for C6H14; give their line-angle formulas & bond positions, 1°, 2°, 3°

! Identify all primary, secondary, tertiarycenters by writing in 1°, 2°, 3° next to eachcarbon

Practice problem

1°2°

2° 2°2°

1°1°

1°

3°2°

2°1°

3°3°

3°2° 2°1°

1°

1°

1°

1°

1°1°

1°1°

1° 1°

2°4°

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This problem may be turned in, sodo it an place it in your portfolio! Draw all fourteen isomers for C5H12O

! give their line-angle formulas.! Identify all primary, secondary, tertiary centers by

writing in 1°, 2°, 3° next to each carbon! In each isomer, circle and name each alkyl branch

attached to an oxygen atom.! For example

The answer is linked on the bond position web page.

O

butyl methyl

1° 2°2° 1°

1°

Called methyl butyl ether

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Additional practice problems

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3.4 Naming Alkanes (IUPAC)

! Compounds are given systematic names by aprocess that uses! Prefix-Parent-Suffix

! Due Process follows specific rules! Circle longest continuous chain! Number to the closest branch! If two possibilities-chose one with more branches! Carbons in that chain are numbered in sequence! Substituents are numbered at their point of attachment! Complex substituents are named as compounds would

be

! See specific examples in text

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Good practice problems

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The IUPAC rules can be very complex,but each organic compound can be assigned a uniquename, and from the name the structure can be written.

Many organic compounds are known bytheir common or trivial name – especiallyif they are complex. These frequently give no clue as tothe structure.

Listening for endings, like, -ol for alcohol, -one for ketone,-al for aldehyde, -ane for alkane, -ene for alkene,-yne for alkyne, alkyl -ate for ester are helpful inidentifying the major functional group in a molecule

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3.5 Properties of Alkanes

! Called paraffins (low affinity compounds)because they undergo few types of reactions

! Complete combustion produces CO2carbon dioxide, water, and heat

! They react with Cl2 in the presence of light toreplace H’s with Cl’s (not controlled)

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Physical Properties

! Boiling points and melting points increase as size ofalkane increases

! Forces between molecules (dispersion and inducedtemporary dipoles if cooled & compressed) are weak

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3.6 Cycloalkanes

! Cycloalkanes are alkanes that have carbon atomsthat form a ring (called alicyclic compounds)

! Simple cycloalkanes rings of %CH2% units, (CH2)n,or CnH2n

! Structure is shown as a regular polygon with thenumber of vertices equal to the number of C’s (aprojection of the actual structure)

cyclopropanecyclohexanecyclopentane

cyclobutane

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Complex Cycloalkanes

! Naturally occurring materials contain cycloalkane structures! Examples: chrysanthemic acid (cyclopropane),

prostaglandins (cyclopentane), steroids (cyclohexanes andcyclopentane)

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Properties of Cycloalkanes

! Melting points are affected by the shapes and theway that crystals pack so they do not changeuniformly

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3.7 Naming Cycloalkanes

! Count the number of carbon atoms in the ring and the number inthe largest substituent chain. If the number of carbon atoms inthe ring is equal to or greater than the number in the substituent,the compound is named as an alkyl-substituted cycloalkane

! For an alkyl- or halo-substituted cycloalkane, start at a point ofattachment as C1 and number the substituents on the ring sothat the second substituent has as low a number as possible.

! Number the substituents and write the name! See text for more details and examples

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Good practice problems

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3.8 Cis-Trans Isomerism inCycloalkanes! Rotation about C-C bonds in cycloalkanes is limited by the

ring structure! Rings have two “faces” and substituents are labeled as to

their relative facial positions! There are two different 1,2-dimethyl-cyclopropane isomers,

one with the two methyls on the same side (cis) of the ringand one with the methyls on opposite sides (trans)

&, Beta (top)above face

', alpha (bottom)below face

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Stereoisomers

! Compounds with atoms connected in the same order but whichdiffer in “fixed” three-dimensional orientation, are stereoisomers

! The geometric isomer terms “cis” and “trans” should be used tospecify stereoisomeric ring structures

! Recall that constitutional isomers have atoms connected indifferent order

&, Beta (top)above face

', alpha (bottom)below face

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Good practice problems