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Chapter 2: Alkanes

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I. Classes of Organic Compounds

Hydrocarbons (C & H only) Heteroatomic compounds

aliphatic aromatic

alkanes

alkenes

alkynes

cyclic compounds

alcohols

ethers

aldehydes

ketones

carboxylic acids

esters

amines

amides

C C

C C

C C

R OH

R O R'

R C

O

H

R C

O

R'

R C

O

OH

R CO

OR'

R C

O

NH2

R NH2

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II. Alkanes

A. General formula: CnH2n + 2

CH4 methane (natural gas, swamp gas)

CH3CH3 (C2H6) ethane

CH3CH2CH3 (C3H8) propane (LPG)

CH3CH2CH2CH3(C4H10) butane

75-85% CH4

5-10% C2H6

1-5% C3H8

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II. Alkanes

B. Isomers: normal and branched alkanes

n-butane

straight chain

branched chain

isobutane

CH3 CH2 CH2 CH3

CH3

CH CH3

CH3

H C C C C H

H H

H H

H H

H H

constitutional

isomers

C4H10

butane lighters:

5% n-butane95% isobutane

Chem3D

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II. Alkanes


C5H12 n-pentane

isopentane

neopentane

CH3 CH2 CH CH3

CH3

CH3 C

CH3

CH3

CH3

CH3 CH2 CH2 CH2 CH3

C6H14 5 isomers

C10H22 75 isomers

C20H42 366,319 isomers

obviously need system of nomenclature

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II. Alkanes


Question 2-1.Draw the constitutional isomers of the following. Click onthe arrow to check your answers.

C6H14(5)

C7H16(9)

CheckAnswer

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II. Alkanes


Answer 2-1.Draw the constitutional isomers of the following. Click onthe arrow to check your answers.

C6H14(5)

C7H16(9)

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II. Alkanes


Types of carbons:

CH3 CH

CH3

CH2 CH3

methyl carbons

methylene carbon

methine carbon

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II. Alkanes

C. Structure-property relationships

Intermolecular attractions: London (or van der Waals) forces (weak)b.p.

CH4 -160C

C2H6 -89

C3H8 -42

n-C4H10 -0.4n-C5H10 36

Branching: lowers b.p.

-0.4C -10.2C

36C 28C 9C

vs

greater surface area

for attraction

Chem3D

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III. IUPAC Nomenclature

A. Parent chains: normal alkanes

Parent names:

CH4 methane n-C11H24 undecane

CH3CH3 ethane n-C12H26 dodecane

CH3CH2CH3 propane n-C13H28 tridecaneCH3(CH2)2CH3 butane n-C14H30 tetradecane

CH3(CH2)3CH3 pentane

CH3(CH2)4CH3 hexane n-C20H42 icosane

n-C7H16 heptane n-C30H62 triacontane

n-C8H18 octane

n-C40H82 tetracontanen-C9H20 nonane

n-C10H22 decane etc.

systematic name: {side groups}parent chain{suffix}

know

to here

(teens)

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B. Substituents: alkyl groups

CH3

CH3 CH2 CH2

CH3CH2CH2CH2

CH3 CH CH2

CH3

CH3 CH2

CH3 CH CH3

CH3CH2CHCH3

CH3 C CH3

CH3

methyl group ethyl group

propyl isopropyl

butyl sec-butyl

isobutyl tert-butyl

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C. IUPAC rules

1. Find the longest straight chain = parent chain

2. Number the parent chain in the direction that gives the lowest number

to the substituents at the first point of difference.

3. Two or more identical groups are indicated by di, tri, tetra, etc.

CH3 CH2 CH2 CH2 CH

CH2 CH3

CH3 Longest chain is 7 (not 6).This is 3-methylheptane.

.

2,3,6-trimethylheptane (not 2,5,6-trimethyl

heptane)

2,2,6,6,7-pentamethyloctane (not

2,3,3,7,7-pentamethyloctane)

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4. Different groups are listed in alphabetical order (di, tri, tetra, etc.dont count; n,sec, tertdont count; iso does).

5. If the numbering is the same in both directions, choose the

numbering to follow the alphabetical order.


C. IUPAC rules

6-ethyl-5-isopropyl-2,2-dimethyloctane

5-ethyl-6-methyldecane

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6. When two or more chains compete for the longest, the choice goes tothe one with the greater number of side groups.

7. Complex side groups (usually > 4 carbons) are named systematicallya. start numbering at the carbon attached to the parent chain

b. enclose name in parentheses

c. first letter counts alphabetically, regardless of what it is


C. IUPAC rules

2,4,6-trimethyl-5-propyloctane

4-(1,2-dimethylpropyl)-3-(1-methylethyl)-

decane

1

2

3

1

2

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Question 2-3.Name the following compounds. Click the arrow tocheck your answers.


C. IUPAC rules

CheckAnswer

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Answer 2-3.The names are here.


C. IUPAC rules

6-ethyl-2,2,5,7-tetramethylnonane (not

6,7-diethyl-2,2,5-trimethyloctane.Did we catchyou? :)

8-sec-butyl-7-tert-butyl-4-isopropyldodecane

(hyphenated words arent alphabetized).

OR

7-(2,2-dimethylethyl)-4-(1-methylethyl)-

8-(2-methylpropyl)dodecane

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8. Cycloalkanes


C. IUPAC rules

CH2

CH2H2C = cyclopropane

general

formula:

CnH2n

ethylcyclobutane

1-isopropyl-3-methylcyclopentane

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Question 2-4. Name the following cycloalkanes. Check your answersby clicking on the arrow.


C. IUPAC rules

CheckAnswer

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Answer 2-4. Name the following cycloalkanes. Check your answersby clicking on the arrow.


C. IUPAC rules

1,1,2-trimethycyclohexane

4-cyclopentyl-2,3-dimethylhexane

1,2-dicyclopropylethane

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IV. Chemical Properties of Alkanes

A. Combustion

C8H18 +

Heats (enthalpies) of combustion: DHcomb

More branchedisomers have lower DHcomb

, are more stable.

1307.5 kcal 1306.3 kcal 1304.6 kcal 1303.0 kcal~ ~ ~ ~

~~~~

CO2+ H2O

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B. Oxidation and reduction in organic molecules

CH4 + Br2 CH3Br + HBr

C HH

H

HC HH

OH

HC

O

H HC

O

H OHC

O

O

oxidation

more C-O bonds, fewer C-H bonds

reduction

fewer C-O bonds, more C-H bonds

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B. Oxidation and reduction in organic molecules

H3C CH3 H2C CH2 HC CH

oxidation

fewer C-H bonds

reduction

more C-H bonds

H2C=CH2+ H2O2HOCH2CH2OH

H2C=CH2+ H2OCH3CH2OH

H2C=CH2+ H2CH3CH3

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B. Assigning oxidation numbers:

1. For each atom in a bond, assign a +1 to the more

electropositive atom and a -1 to the more electronegative

atom. (If the atoms are the same, each atom gets a 0.)

2. Total each atom.

C O

Cl-1+1

-1+1

H

-1

-1

+1

+1

C O

Cl-1+1

-1+1

H

-1

-1

+1

+1 Oxidation number of carbon is

+1 +1 +1 -1 = +2

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Question 2-5. Determine oxidation numbers for the carbons

in each of the following molecules. Click on the arrow to check

your answers.

CheckAnswer

H C N

C

H

H

Br

C

O

O H12

C

H

H

OH

Cl

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Answer 2-5. Determine oxidation numbers for the carbons

in each of the following molecules. Click on the arrow to check

your answers.

H C N

C

H

H

Br

C

O

O H12

C

H

H

OH

Cl

C = +2

C1= +3; C2= -1

C = 0

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Question 2-6. Determine whether the following reactions will

be oxidation, reduction, or neither. Check your answers by clicking

on the arrow.

CheckAnswer

H2C CH2 + Br2 H2C CH2

Br Br

H2C CH2 + H2O C

H

H

H

C

H

H

OH

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Answer 2-6. Determine whether the following reactions will

be oxidation, reduction, or neither. Check your answers by clicking

on the arrow.

CheckAnswer

H2C CH2 + Br2 H2C CH2

Br Br

H2C CH2 + H2O C

H

H

H

C

H

H

OH

Oxidation: Each carbon

changes from a -2 to a -1.

Neither. One carbon changes from a -2 to a -3 (gets

reduced), but the other carbon goes from a -2 to a -1

(gets oxidized.) No net change.

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6.alkanes