Chapter 2: Alkanes

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Alkanes from Carbon and Hydrogen•Alkanes are carbon compounds that contain only single bonds.

•The simplest alkanes are hydrocarbons – compounds that contain only carbon and hydrogen.

•Hydrocarbons are used mainly as fuels, solvents and lubricants:

# of carbons boiling point range Use

1-4

5-6

6-12

12-15

15-18

16-24

<20 °C

30-60

60-90

85-200

200-300

>400

300-400

6-7

fuel (gasses such as methane, propane, butane)

solvents (petroleum ether)

solvents (ligroin)

fuel (gasoline)

fuel (kerosene)

fuel (heating oil)

lubricating oil, asphalt

C C C C H

H

H

H

H

H

H

H

H

H

C C C C C

H

H

H

CH2

H

H

H

H

H

H

CH3

CH3

H C C

CC

C

HH

H

HH H

H

H

HH

Hydrocarbons

CH4

C2H6

C3H8

C4H10

methane

ethane

propane

butane

C6H14

C7H16

C8H18

C9H20

C10H22

hexane

heptane

octane

nonane

decane

Prefix Suffix

meth-

eth-

prop-

but-

hex-

hept-

oct-

non-

dec-

-ane

-ane

-ane

-ane

-ane

-ane

-ane

-ane

-ane

Formula Name StructureH

C

H

HH

H3C CH3

C5H12 pentanepent- -ane

Hydrocarbons

CH4

C2H6

C3H8

C4H10

methane

ethane

propane

butane

Prefix Suffix

meth-

eth-

prop-

but-

-ane

-ane

-ane

-ane

Formula Name Structure

H3C C CH3

H

H

H3C C C CH3

HH

H H

H

C

H

HH

or

or

H

C

H

CH H

H

H

C4H10 butane?but- -ane H3C C CH3

CH3

H

or

CH4

C2H6

C3H8

C4H10

methane

ethane

propane

butane

Prefix Suffix

meth-

eth-

prop-

but-

-ane

-ane

-ane

-ane

Formula Name Structure

H3C C CH3

H

H

H3C C C CH3

HH

H H

H

C

H

HH

or

or

H

C

H

CH H

H

H

C4H10 but- -ane H3C C CH3

CH3

H

or

Hydrocarbons

iso-butane

Hydrocarbons

C4H10 butane

Prefix Suffix

but- -ane

Formula Name Structure

H3C C C CH3

HH

H H

or

C4H10 iso-butanebut- -ane H3C C CH3

CH3

H

or

C5H12 pentanepent- -ane H3C C C C

HH

H H

H

H

CH3 or

H3C C C C

CH3H

H H

H

H

H

H C C C

CH3H

H CH3

H

H

H

or

or

C5H12 iso-pentanepent- -ane

C5H12 iso-pentane???pent- -ane

HydrocarbonsHydrocarbons

C4H10 butane

Prefix Suffix

but- -ane

Formula Name Structure

H3C C C CH3

HH

H H

or

C4H10 iso-butanebut- -ane H3C C CH3

CH3

H

or

C5H12 pentanepent- -ane H3C C C C

HH

H H

H

H

CH3 or

H3C C C C

CH3H

H H

H

H

H

H C C C

CH3H

H CH3

H

H

H

or

or

C5H12 iso-pentanepent- -ane

C5H12 neo-pentanepent- -ane

HydrocarbonsHydrocarbons

IUPAC System of Naming Organic Compounds

pentaneH3C C C C

HH

H H

H

H

CH3

H3C C C C

CH3H

H H

H

H

H

H C C C

CH3H

H CH3

H

H

H

2-methylbutane

2,2-dimethylpropane

1. Find the longest carbon chain

2. Name all the atoms (other than H) not in the longest chain as groups

3. Number groups from the end of the chain closest to the first group

4. List multiple groups in alphabetical order,

1

2

3

4

5

Longest chain: pentane

Groups: none

IUPAC Name

12

34 Longest chain: butane

Groups: one methyl

12

34

not:

not: 3-methylbutane

12

3Longest chain: propane

Groups: two methyls

C4H10 butane

Prefix Suffix

but- -ane

Formula Structure

H3C C C CH3

HH

H H

or

C4H10 iso-butanebut- -ane H3C C CH3

CH3

H

C5H12 pentanepent- -ane H3C C C C

HH

H H

H

H

CH3

H3C C C C

CH3H

H H

H

H

H

H C C C

CH3H

H CH3

H

H

H

C5H12 pent- -ane

C5H12 pent- -ane

Hydrocarbons

IUPAC Name

2-methyl butane

2,2-dimethylpropane

IUPAC System of Naming Organic Compounds

–CH3

–C2H5

–C4H9

methyl

ethyl

butyl

Prefix Suffix

meth-

eth-

but-

-yl

Group Formula Name Structure

C C C CH3

HH

H H

H

C

H

H

or

H

C

H

C H

H

H

-yl

-yl

H

H

–C3H7 propylprop- C C CH3

H

H

or-yl

H

H

–C3H7 iso-propyliso-prop-H3C C CH3

H

or-yl

Abbreviation

–Me

–Et

–Pr

–Bu

–iPr

Naming Branches as GroupsNaming Branches as “Groups”

–C4H9

sec-butyl

Group Formula Name Structure

–C4H9

iso-butyl

–C4H9 tert-butyl

Other Groups We May Encounter

–C5H11 neo-pentyl

–F

–Cl

–Br

–I

bromo

fluoro

chloro

iodo

–F

–Cl

–Br

–I

Group Formula Name Structure

Naming Branches as “Groups”

Structural IsomersHow many structural isomers – compounds with the same chemical formula but different connectivity – are there with the formula C7H16?

Structural IsomersHow many structural isomers – compounds with the same chemical formula but different connectivity – are there with the formula C7H16?

CH3

CH3

CH3 CH3

CH3

CH3

IUPAC Name

1

2

3

4

5

12

34

5

1

2

3

4

5

2,2-dimethylpentane

2,4-dimethylpentane

H3C

CH3

12

3

4

5

1

23

4

5

3,3-dimethylpentane

3-methylhexane

3-ethylpentane

12

34 5 6

––

–

–

–

CH3

CH3

12

34

52,3-dimethylpentane

IUPAC Name

2-ethylpentane

CH3

CH3

1

2

3

4

5

6

7heptane

12

3

4

5

6

1

23

4

5

6

2-methylhexane

3-methylhexane

123

456

CH3

1

2

34

5

6

–

CH3

1

2

34

5

6–

––

–

–

4-methylhexane

3-methylhexane

1

2

3

42,2,3-Trimethyl-butane

H3C– methyl

Group Formula Name StructureH

C

H

H

Abbreviation

–Me

Methyl & Ethyl Compounds

H3C–H

H3C–OH

H3C–NH2

H3C–F

H3C–Cl

H3C–Br

H3C–I

H3C–CN

H3C–SH

methane

methyl alcohol (methanol)

methylamine (aminomethane)

methyl fluoride (fluoromethane)

methyl chloride (chloromethane)

methyl bromide (bromomethane)

methyl iodide (iodomethane)

methyl cyanide (cyanomethane, acetonitile)

methanethiol (methyl mercaptan)

H5C2– ethyl

H

C

H

C H

H

H

–Et

H5C2–H

H5C2–OH

H5C2–NH2

H5C2–F

H5C2–Cl

H5C2–Br

H5C2–I

H5C2–CN

H5C2–SH

ethane

ethyl alcohol (ethanol)

ethylamine (aminoethane)

ethyl fluoride (fluoroethane)

ethyl chloride (chloroethane)

ethyl bromide (bromoethane)

ethyl iodide (iodoethane)

ethyl cyanide (cyanoethane, proprionitile)

ethanethiol (ethyl mercaptan)

Methyl and Ethyl Compounds

H3C– methyl

Group Formula Name StructureH

C

H

H

Abbreviation

–Me

Methyl & Ethyl Compounds

H3C–H

H3C

H3C

H3C

methane

methyl cation

methyl radical

methyl anion

H5C2– ethyl

H

C

H

C H

H

H

–Et

C HH

H

CH

HH

CH

HH

H5C2–H

H3C–CH2

H3C–CH2

H3C–CH2

ethane

ethyl cation

ethyl radical

ethyl anion

C HH3C

H

CH

H3CH

CH

H3CH

Methyl and Ethyl Compounds

Group Formula Name StructureAbbreviation

prop-

H7C3–iso-prop-

–Pr

–iPr

H7C3– C C CH3

H

H

H

H

H3C C CH3

H

or

or

Propyl Compounds

propyl alcohol (1-propanol)

propyl amine (1-aminopropane)

propyl fluoride (1-fluoropropane)

propyl chloride (1-chloropropane)

propyl bromide (1-bromopropane)

propyl iodide (1-iodopropane)

isopropyl alcohol (isopropanol; 2-propanol)

isopropyl amine (2-aminopropane)

isopropyl fluoride (2-fluoropropane)

isopropyl chloride (2-chloropropane)

isopropyl bromide (2-bromopropane)

isopropyl iodide (2-iodopropane)

OH

NH2

F

Cl

Br

I

12

3 HO

H2N

F

Cl

Br

I

1

23

Propyl Compounds

Structural Isomers of C7H16

Cycloheptane contains two fewer hydrogens than heptane. They are not structural isomers.

12

3

4

5

67 heptaneWhat if we attatch the two ends together?

cycloheptane

12

3

45

6

7

H2C CH2

CH2

CH2

H2C

H2C

H2C

C7H14

Cycloalkanes

cyclopropane

cyclobutane

cyclohexane

cycloheptane

cyclooctane

Compound Name Structure

cyclopentane

cyclopropyl

cyclobutyl

cyclohexyl

cycloheptyl

cyclooctyl

cyclopentyl

Group Name Structure

Cycloalkanes

Naming Cycloalkanes

The above compounds are not structural isomers. Their connectivity is identical. They are stereoisomers – compounds that differ in how substituents are arranged in space

Unlike "floppy" chains, rings have an additional consideration for multiple substitutions:

Me

HH

H

Me

H

"flat" 1,2-dimethylcyclopropane

Me Me

HH

Me

H

H

"3-D" 1,2-dimethylcyclopropane

These are different compounds! They cannot be interconvertied by bond rotation.

Me

"3-D" 1,2-dimethylcyclopropane

Me

HH

H

Me

HMe

HH

Me

H

H

cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane

cis – substituents on same side (of the ring)

trans – substituents on opposite sides (of the ring)

IUPAC Naming of Rings

How can we represent cis/trans isomers in a flat drawing?

Me

HH

H

Me

HMe

HH

Me

H

H

cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropaneMe

Me

Me

Me

A solid wedge means the bond is pointing "up" and out of the page A dashed wedge means that the bond

is pointing "down" and into the page

Naming Cycloalkanes

IUPAC Naming of Rings

cis-1,2-dimethylcyclopropane

Me

Me

Me

HH

Me

H

H

Me

Me

H

MeH

H

Me

H

Don't let the wedges and dashes confuse you:

cis-1,2-dimethylcyclopropane

Me

Me

Me

Me

These are the same compound!

Flip over 180°

Naming Cycloalkanes

3-Dimensional Structure/VSEPR Theory

X

XX

A

XA = any atom

X = any substituent (atom or lone pair)

X

XX

A

X

HH

N

H

H

H

O

H

HH

C

H

F

FF

B

F

109°

Atoms place substituents as far appart as possible:

tetrahedral electron-domain geometries

A dashed wedge means: the substituent is behind the plane of the paper

A solid wedge means: the substituent is in front of the plane of the paper

3-Dimensional Structure

C C

H

H

H

H

H

H H

CCHHH

HH

H

H

H

H

C

H

C

H

"flat" ethaneeach carbon is tetrahedral

"3-D" ethane

wedge/dash depiction of ethane

H

HH

H

HH

H

HH

H

HH

C C

H H

H HH H

The Conformation of Ethane

H

CCHHH

HH

If we look down the C–C bond axis, we get a different perspective:

C

H

HH

H

HH

H

CCHHH

HH

But there is free rotation around single (σ) bondσ:

theσe hydrogenσ are in the σame plane

theσe hydrogenσ are in the σame plane

W e can uσe a Newm an Projection to depict thiσ view:

Front atom appearσ at vertex here

The large circle iσ a σtylized repreσentation of the back atom

Newm an Projection

H

HH

H

HH

The "σtaggered" conform ation The "eclipσed" conform ation

3-Dimensional Structure

3-Dimensional Structure

Molecules that differ by only rotations around single bonds are conformational Isomers.

H

HH

H

HH

C C

H H

H HH H

H

CCHHH

HH

these hydrogens are in the same plane

these hydrogens are in the same plane

H

HH

H

HH

The "staggered" conformation The "eclipsed" conformation

H

HH

H

HH

The energy minimum

The staggered conformation is the lowest energy conformation

H

HH

H

HH

The energy maximum

The elclpsed conformation is the highest energy confornation

How can these molecular shapes arise from bonding between the available orbitals?

180°

C NH3C

Place substituents as far appart as possible:

Molecule is linear

H

HH

C

H

109°

F

F

B

F

120°

Molecule is trigonal planar

Molecule is tetrahedral

2s 2px

2nd row valence orbitals

2py 2pz

H3CCN

BF3

CH4

3-Dimensional Structure

Hybridization

C C

this side gets bigger and 50% more spherical this side gets smaller and

50% more spherical

mixingCand

2s

2px

2 unhybridized orbitals 2 sp-hybridized orbitals

Only "X"-directionality; 50% s-like and 50% p-like

180°

C NH3C

Molecule is linear

2s 2px

2nd row valence orbitals

2py 2pz

H3CCN

C NH3C C NH3C

sp-orbitals on the central carbon atom the σ-bond form ed by σp-orbital overlap

C NH3C

p-bonding in the triple bond

C C

this side gets bigger and 33% more spherical this side gets smaller and

33% more spherical

mixing and

2s

2px

3 unhybridized orbitals 3 sp-hybridized orbitals

Only "X" and "Y"-directionality; 33% s-like and 67% p-like

2s 2px

2nd row valence orbitals

2py 2pz

F

F

B

F

120°

Molecule is trigonal planar

BF3

2py

C

C

and

B

F

F

All 3 sp2-orbitals on the boron atom The σ-bondσ between boron and fluorine

F B

F

F

F

Hybridization

mixing

All 4 unhybridized orbitals Each orbital has "X", "Y" and "Z"-directionality and are 25% s-like and 75% p-like

2s 2px

2nd row valence orbitals

2py 2pz

All 4 sp3-orbitals on the carbon atom The σ-bondσ between carbon and hydrogen

H

HH

C

H

109°

Molecule is tetrahedral

CH4

H

HH

C

H

H

HH

C

H2s 2px 2py 2pz

Hybridization