Chemical Reactions of Alkanes

1. Combustion

2. Desaturation- forms alkenes

3. Halogenation- forms alkyl halides

Chemical Reactions of Alkanes: Combustion

CH4 + 2 O2 CO2 + 2 H2O + energy

2 C8H18 + 25 O2 8 CO2 + 9 H2O + energy

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.

Chemical Reactions of Alkanes: Desaturation- forms alkenes

.

.

C C

H

H

H

H H

H

Pd catalyst

H2

C CH H

HH

Chemical Reactions of Alkanes: Halogenation- forms alkyl halides

.

.

C C

H

H

H

H H

H

C C

H

H

H

H H

ClCl2 HCl

h

Mechanism of Halogenation is Free Radical Based

.

Cl2 2 Clh

.

.

Cl Clh

2 Cl

.

.

C C

H

H

H

H

H

C C

HH

H

H

H

H Hh

.

+

2nd Step in Mechanism of Halogenation is Elongation

.

C C

H

H

H

H

H

C C

H

H

H

H H

H

C C

HH

H

H

H

Cl

Cl

.

HCl

or

C C

H

H

H

H

H ClCl2

Various Combinations of Free Radicals Represent Termination

.

.

C C

H

H

H

H

H

C C

H

H

H

H C

H

C

H

H H

H

H

C C

HH

H

H

H

Cl

.

or

C C

H

H

H

H

H Cl

+ C C

H

H

H

H

H

or . . .

Nomenclature of Alkyl Halides

1. Chain as alkanes

2. -ine of halogen changed to -o

3. Identify attachment site

e.g. 2-chloroethane

Physical Properties of Alkyl Halides

1. Polar

2. Liquid instead of gas

3. Tend to be toxic, taken internally

Predominant Product

Determined by radical stability

Direction of stability

Stereochemistry

C

H

CH2CH3

CH3 Br C

Br

CH2CH3

CH3 H

.

.

• Chiral Carbon- a carbon which has four different groups bonded to it

• Dashed wedge: behind; solid wedge, in front• Enantiomers- nonsuperimposable, mirror images

The RS system of nomenclature

CH CH3

CH2CH3

Cl

.

.

1. Position the molecule so the H is in back

2. Number highest mol. wt. (1) to lowest (3)

3. Clockwise R, counterclockwise S

Alkenes: Names and Structures

Alkenes- hydrocarbons that have one or more double bonds between carbon atoms

CnH2n

Monounsaturated- one double bond

Polyunsaturated- two or more double bonds

Naming Alkenes

1. Find the longest continuous carbon chain that contains the double bond(s), even if it is not the longest carbon chain, and number from the end nearer the double bond.

2. Write the alkane name corresponding to the longest carbon chain, then change the -ane suffix to -ene.

3. Identify by number the lower-numbered carbon atom containing a double bond, followed by a hyphen directly before the alkene name; precede this by naming substituents in the usual way.

4. If two or more double bonds occur, specify each double bond by number and use an appropriate prefix before -ene to indicate the number of double bonds.

Cis-Trans Isomerism

1. “R groups” must be attached to different carbons

2. Larger “R groups” get priority

C

C

H3C H

HH3C

C

C

H CH3

HH3C

C

C

H3C CH3

HH

Practice

C C

CH3

CH3

H

CH3

.

.

C C

CH3CH2CH2

H

CH2CH2CH3

H

.

.

• Give the IUPAC name for the above molecules

More Practice

• Draw the structure of the following compounds, for which a correct IUPAC name is given:

cis-3-hexene

3-ethylcyclohexene

C C

CH3CH2

H

CH2CH3

HCH2CH3 .

.

Properties of Alkenes and Alkynes

1. nonpolar

2. less dense than alkanes

3. trend toward gas rather than liquid

4. used to make polymers

Reactions of Alkenes

Addition reaction- since H has been taken away to create a double bond, H or other molecules can be added across the double bond, yielding a substituted alkane.

1. catalytic hydrogenation

2. halogenation

3. addition of HCl and HBr (Markovnikonv's rule)

4. hydration

5. oxidation

Reactions of Alkenes: Catalytic Hydrogenation

.

.

C C

H

H

H

H H

H

Pd catalyst

H2

C CH H

HH

Reactions of Alkenes: Halogenation

.

.

C C

H

Br

H

H H

Cl

C CH H

HH

Br2 or Cl2

C CH H

HHBr Br C C

H H

HH

BrBr

Reactions of Alkenes: Acid-Catalyzed Hydration (Markovnikov’s Rule)

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.

CH3CH2CH CH3HCH3CH2CH CH2 +

O H

H

CH3CH2CH CH3 + CH3CH2CH CH3

OH

H+

Reactions of Alkenes: Addition of HCl and HBr (Markovnikov’s Rule)

CH3CH CH3

Br

CH3CH CH3 + Br

CH3CH CH2 H Br+ CH3CH CH3 + Br

.

.

Reactions of Alkenes: Catalytic Hydrogenation

Addition reaction- since H has been taken away to create a double bond, H or other molecules can be added across the double bond, yielding a substituted alkane.

1. catalytic hydrogenation

2. halogenation

3. addition of HCl and HBr (Markovnikonv's rule)

4. hydration

5. Others exist

Alkynes: Names and Structures

Alkynes- hydrocarbons that have one or more triple bonds between carbon atoms

CnH2n-2

Named as alkenes, with the exception that -yne is used instead of -ene

Reactions of Alkynes

Since H has been taken away to create the triple bond, just as for a double bond, the same reactions will occur for alkynes as alkenes.

The difference is, 2 moles will be added instead of 1.