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CH 3: Organic Compounds: Alkanes and Their Stereochemistry

Renee Y. Becker

CHM 2210

Valencia Community College

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Why this Chapter

• Alkanes are unreactive, but provide useful vehicle to introduce important ideas about organic compounds

• Alkanes will be used to discuss basic approaches to naming organic compounds

• We will take an initial look at 3-D aspects of molecules

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Functional Groups

• Functional group - collection of atoms at a site that have a characteristic behavior in all molecules where it occurs

• The group reacts in a typical way, generally independent of the rest of the molecule

•For example, the double bonds in simple and complex alkenes react with bromine in the same way

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Functional Groups with Multiple Carbon–Carbon Bonds

• Alkenes have a C-C double bond

• Alkynes have a C-C triple bond

• Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring & bonds?

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Functional Groups with Carbon Singly Bonded to an Electronegative Atom

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Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

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8

9

10

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Alkanes

• Alkanes: Compounds with C-C single bonds and C-H bonds 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

• They are also called aliphatic compounds

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Naming Alkanes

Memorize 1-10 for next class

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Alkane Isomers

• The molecular formula of an alkane with more than three carbons can give more than one structure– C4 (butane) = butane and isobutane

– C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane

• Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes

• Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

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Constitutional Isomers

• Isomers that differ in how their atoms are arranged in chains are called constitutional isomers

• Compounds other than alkanes can be constitutional isomers of one another

• They must have the same molecular formula to be isomers

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Condensed Structures of Alkanes

• We can represent an alkane in a brief form or in many types of extended form

• A condensed structure does not show bonds but lists atoms, such as – CH3CH2CH2CH3 (butane)– CH3(CH2)2CH3 (butane)

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Alkyl Groups

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

• General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule)

• Name: replace -ane ending of alkane with -yl ending

– CH3 is “methyl” (from methane)

– CH2CH3 is “ethyl” from ethane

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Types of Alkyl groups

• Classified by the connection site (See Figure 3.3)– a carbon at the end of a chain (primary alkyl group)– a carbon in the middle of a chain (secondary alkyl

group)– a carbon with three carbons attached to it (tertiary

alkyl group)

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Types of Hydrogens

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Substituents

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Common Names

• Isobutane, “isomer of butane”

• Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain.

• Neopentane, most highly branched

• Five possible isomers of hexane,18 isomers of octane and 75 for decane!

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Pentanes

CH

H

C

H

H

C

H

H

C

H

C

HH

H

H

H

n-pentane, C5H12

CHH

C CHH

C H

H

H

HH

CH

HH

isopentane, C5H12

C

CH3

CH3H3C

CH3

neopentane, C5H12

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IUPAC Names

• Find the longest continuous carbon chain.

• Number the carbons, starting closest to the first branch.

• Name the groups attached to the chain, using the carbon number as the locator.

• Alphabetize substituents.

• Use di-, tri-, etc., for multiples of same substituent.

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Longest Chain

• The number of carbons in the longest chain determines the base name: ethane, hexane.

• If there are two possible chains with the same number of carbons, use the chain with the most substituents.

C

CH3

CH2

CH3

CH CH2 CH2 CH3

CH CH2 CH3H3C

H3C

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Number the Carbons

• Start at the end closest to the first attached group.

• If two substituents are equidistant, look for the next closest group.

1

2

3 4 5

6 7CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

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Name Alkyl Groups

• CH3-, methyl

• CH3CH2-, ethyl

CH3 CH CH2 CH3

sec-butyl

CH3 CH

CH3

CH2

isobutyl

CH3 CH CH3

isopropyl

CH3C

CH3

CH3

tert-butyl

• CH3CH2CH2-, n-propyl

• CH3CH2CH2CH2-, n-butyl

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Propyl Groups

C

H

H

H

C

H

H

C

H

H

H

n-propyl

C

H

H

H

C

H

C

H

H

H

isopropyl

H

A primary carbon A secondary carbon

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Butyl Groups

C

H

H

H

C

H

C

H

H

C

H

H

H

C

H

H

H

C

H

C

H

HH

C

H

H

n-butyl sec-butyl

H

H

A primary carbon A secondary carbon

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Isobutyl Groups

CH

H

H

C

CH H

C

HH

H H

CH

H

H

C

CH H

C H

HH

HH

H

A primary carbon A tertiary carbon

isobutyl tert-butyl

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Alphabetize

• Alphabetize substituents by name.

• Ignore di-, tri-, etc. for alphabetizing.

CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

3-ethyl-2,6-dimethylheptane

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Example 1

Write structures for the following:

a) 3-ethyl-3-methylpentane

b) 4-t-butyl-2-methylheptane

c) 5-isopropyl-3,3,4-trimethyloctane

d) 3-ethyl-2,4,5-trimethylheptane

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Example 2

Provide IUPAC & common names (1-3)

1

2

3

4

5

6

1

2

3

4

5

6

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Example 3Name the following

a) 3,3-dimethyl-4-isobutylheptane

b) 3,3-dimethyl-4-tert-butylheptane

c) 4-tert-butyl-3,3-dimethylheptane

d) 4-isobutyl-3,3-dimethylheptane

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Complex Substituents

• If the branch has a branch, number the carbons from the point of attachment.

• Name the branch off the branch using a locator number.

• Parentheses are used around the complex branch name.

• For alphabetizing use the first letter of the complex sub. Even if it is a numerical (di, tri, etc)

Complex Examples

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a

b

c

d

a

b

c

d

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Example 4

Draw the structures and give their more common names

a) (1-methylethyl) group

b) (2-methylpropyl) group

c) (1-methylpropyl) group

d) (1,1-dimethylethyl) group

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Example 5

Draw the structures

a)4-(1-methylethyl)heptane

b) 5-(1,2,2-trimethylpropyl)nonane

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Assignment

• For next class draw the 9 isomers of heptane and name them

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Properties of Alkanes

• Called paraffins (low affinity compounds) because they do not react as most chemicals

• They will burn in a flame, producing carbon dioxide, water, and heat

• They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

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Physical Properties: Alkanes

• Solubility: hydrophobic

• Density: less than 1 g/mL

• Boiling points increase with increasing carbons (little less for branched chains).

• Melting points increase with increasing carbons (less for odd-number of carbons).

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Boiling Points of Alkanes

Branched alkanes have less surface area contact,so weaker intermolecular forces.

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Melting Points of Alkanes

Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

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Branched Alkanes

• Lower b.p. with increased branching

• Higher m.p. with increased branching

H

CH3CH

CH3

CH2 CH2 CH3

bp 60°Cmp -154°C

CH3CH

CH3

CHCH3

CH3 bp 58°Cmp -135°C

bp 50°Cmp -98°C

CH3 C

C 3

CH3

CH2 CH3

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Example 6

List each set of compounds in order of increasing boiling point and melting point

a

b

a

b

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Example 7

Which of the following has the highest boiling point?

a) 3-methylpentane

b) 2,2-dimethylbutane

c) Hexane

d) Methane

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Conformers

• Conformation- Different arrangement of atoms resulting from bond rotation

• Conformations can be represented in 2 ways:

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Torsional Strain

• We do not observe perfectly free rotation

• There is a barrier to rotation, and some conformers are more stable than others

• Staggered- most stable: all 6 C-H bonds are as far away as possible

• Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other

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Conformations of Ethane

• Stereochemistry concerned with the 3-D aspects of molecules

• Rotation is possible around C-C bonds in open-chain molecules (not cyclic)

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Ethane Conformers • Eclipsed conformer has highest energy• Dihedral angle = 0 degrees

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Conformational Analysis

• Torsional strain: resistance to rotation.• For ethane, only 3.0 kcal/mol

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Propane Conformers

Note slight increase in torsional strain due to the more bulky methyl group.

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Butane Conformers C2-C3

• Highest energy has methyl groups eclipsed.• Steric hindrance• Dihedral angle = 0 degrees

totally eclipsed

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Butane Conformers (2)

• Lowest energy has methyl groups anti.• Dihedral angle = 180 degrees

anti

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Butane Conformers (3)

• Methyl groups eclipsed with hydrogens• Higher energy than staggered conformer• Dihedral angle = 120 degrees

eclipsed

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Butane Conformers (4)

• Gauche, staggered conformer• Methyls closer than in anti conformer• Dihedral angle = 60 degrees

gauche

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Conformational Analysis

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Higher Alkanes

• Anti conformation is lowest in energy.• “Straight chain” actually is zigzag.

CH3CH2CH2CH2CH3

CH C

CCC

H H H H

H H

H H

HH H

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