ALKANES

&
ALKENES
ALKANES
Reactions of alkanes
alkane + H2SO4 → no reaction (NR)
alkane + NaOH → NR
alkane + Na → NR
alkane + KMnO4 → NR
alkane + H2,Ni → NR
alkane + Br2 → NR
alkane + H2O → NR
(Alkanes are typically non-reactive. They dont react with acids, bases,
active metals, oxidizing agents, reducing agents, halogens, etc.)
Halogenation of alkanes
Alkanes undergo substitution reactions with halogens in the
presence of heat or light. Most widely used are chlorination and
bromination. Fluorination is strongly exothermic while iodination
is endothermic.

Reactivity of halogen – F2 > Cl2 > Br2 > I2

Cl or Br

Chlorination of an alkane is not selective and gives every possible

monochloride.

Bromination is highly regioselective for substitution of tertiary

hydrogens. Major synthetic application is in synthesis of tertiary alkyl
bromides. 4
Chlorination of Methane: Mechanism of Reaction

Chain Termination
Polyhalogenation

Polychlorination
Alkenes
 Hydrogenation of Alkenes
Addition of H2 to the C=C (π-bond) of alkenes.
The reaction must be catalyzed by metals such as Pt, Pd, Ni and Rh.
Exothermic reaction but slow without the catalyst.
H H H H
Pd/C
C C + H H C C
H H
EtOH
H H H H

• The catalysts is not soluble in the reaction media (ethanol), thus this process is
referred to as heterogenous catalysis/ reaction.
C=C bonds of aryl rings are not at all easily reduced by catalytic hydrogenation.
8
Stereochemical aspects of alkene hydrogenation

• syn addition of both H atoms to double bond

• hydrogenation is stereoselective, corresponding to addition to less
crowded face of double bond

A reaction in which a single starting material can give two or

more stereoisomeric products but yields one of them in greater
amounts than the other (or even to the exclusion of the other)
is said to be stereoselective.
The addition of H2 across the -bond of C=C is syn (from the same face of
the double bond).

H CH3
CH3 H2, Pd/C CH3 H

EtOH CH3
CH3 CH3
H H

syn addition
of H2 Not observed

10
 H3C CH3
α-pinene
H H2, cat
Top face of double
bond blocked by
this methyl group H3C

H3C CH3
H3C CH3
H
H

H
H
H
Only this H3C
product CH3
H
is formed
Both products correspond to syn addition of H2
 Heats of Hydrogenation -can be used to compare
relative stabilities of alkenes
Alkene H° (KJ/mol)
H2C=CH2 136

H H
monosubstituted
125 - 126
H3C H

H H
117 - 119
H3C CH3

H CH3
disubstituted 114 - 115
H3C H

H3C H
116 - 117
H3C H
The greater release of heat, the less stable the reactant.
H3C H
trisubstituted 112
H3C CH3

H3C CH3
tetrasubstituted 110
H3C CH3 12
Electrophilic Addition of Halogens to Alkenes

• Addition occurs in a variety of solvents (chloroform, dichoromethane) at RT or

below
• Bromine and chlorine add to alkenes to give 1,2-dihalides (vicinal dihalides)
• F2 addition to alkenes is a violent reaction. Vicinal diiodides tend to loose I2 and
revert to alkenes.
CHCl3

0oC
Mechanism
Br
C C + Br Br C C + Br
Br
Br
C C
C C
Br Br

Br+ adds to an alkene producing a cyclic ion (bromonium ion) in which the
positive charge resides on bromine.

Br- attacks the bromonium ion.

Stereochemistry
Bromonium ion opening is SN2 → Anti Addition of Br2
Conversion of Alkenes to Vicinal Halohydrins
H2O
H2C CH2 + Br2 BrCH2CH2OH
(70%)

H
Cl2
H OH
H2O

H Cl H

only product
Anti addition is observed.
 Regioselectivity
H3C CH3
Br2
C CH2 CH3 C CH2Br
H2O
H3C OH
(77%)

The halogen adds to the carbon having the greater number of hydrogens.
Water attacks the more highly substituted carbon.
 Electrophilic Addition of Hydrogen Halides to Alkenes
H H CHCl3 Br H
C C + H-Br C C
H H
H H -30oC H H
nucleophile electrophile
Addition occurs in a variety of solvents (CHCl3, CH2Cl2, pentane and
benzene) at low T’s.

Reactivity of HX correlates with acidity.

slowest HF << HCl < HBr < HI fastest

 Mechanism
Step 1: Pi electrons of C=C attack the electrophile.
CH3 CH3
_
CH3 C CH CH3 CH3 C CH CH3 + Br
+
H
H Br

Step 2: Nucleophile attacks the carbocation.

CH3 CH3
CH3 C CH CH3 CH3 C CH CH3
+
H Br H
_
Regioselectivity Br

Electrophilic addition of HX across a C=C bond in an unsymmetrical alkene is

regioselective. H will add to the carbon of the double bond with the most H’s and
the X will add to the carbon of the double bond that has the most alkyl groups.
18
(Markovnikov's Rule)
 Carbocation Rearrangements in
Hydrogen Halide Addition to Alkenes
H
Cl H
H3C H
H3C C H H-Cl H3C C H
+ H3C C H
C C C C
C C
H3C H3C H
CH3 H CH3 H H3C H
Cl H

Major product
 Free-Radical Addition of HBr

• In the presence of peroxides (ROOR), HBr adds to an alkene to form

the “anti-Markovnikov” product (light or heat).

• Reaction can be initiated photochemically either with or without

added peroxide.

• Only HBr reacts with alkenes by both electrophilic and free-radical

addition mechanisms.

• HCl and HI always add to alkenes by electrophilic addition and

follow Markovnikov’s Rule.
Chapter 8
 Acid-catalyzed hydration of alkenes
+ H OH
H
C C + H2O C C
alkene
alcohol

• Reverse of dehydration of alcohol

• Use dilute acid medium to drive equilibrium
toward hydration.
50% H2SO4 / H2O

Regioselectivity of hydration follows Markovnikov’s

rule.
 Mechanism for Hydration
H
H
+ +
C C + H O H C C + H2O

H
+
H H O H
+
C C + H2O C C

H
+ H
H O H H O
+
C C + H2O C C + H3O

Carbocation rearrangements are possible.

Chapter 8
Indirect Hydration
Hydroboration-Oxidation
•Anti-Markovnikov product formed
•Syn addition of H and OH
CH3 1) B2H6, THF H
2) H2O2, NaOH, H2O CH3

H
HO

-
(1) BH3 (2) H O22,, OH
H22O -OH
C C C C C C
H BH2 H OH
(1) BH3 . THF
 Epoxidation
• Alkene reacts with a peroxyacid to form an epoxide.
• Commonly used – peroxyacetic acid
peroxybenzoic acid
Peroxyacetic acid is normally used in acetic acid as the
solvent, but epoxidation tolerate a variety of solvents
(CHCl3, CH2Cl2).
O O O
C C + R C O O H C C + R C O H

Epoxidation of alkenes with peroxy acids is a syn addition to the double bond.

cis or trans stereochemistry of the alkene is maintained in the product

(stereospecific).
 Ozonolysis (Oxidative Cleavege)
• Ozone (O3) is a neutral but polar molecule and a powerful
electrophile.
• Reaction with ozone forms an ozonide.
• Ozonides undergo hydrolysis in water to give carbonyl compounds.
Aldehydes are easily oxidized to carboxylic acids during hydrolysis.
• To isolate the aldehyde hydrolysis is done in the presence of a
reducing agent.
1. O3 2. H2O, Zn
or
1. O3, CH3OH 2. (CH3)2S
Chapter 8
Ozonolysis Example

O
H CH3 O3 H CH3
C C C C
CH3 CH3 O O
CH3OH H3C CH3
Ozonide

H O
(CH3)2S CH3
C O O C + CH3 S CH3
H3C CH3
DMSO
 Oxidative Cleavage with KMnO4
• Basic solution of KMnO4 upon heating /acidic KMnO4
• Glycol initially formed is further oxidized.
• Disubstituted carbons become ketones. Monosubstituted carbons become
carboxylic acids. Terminal =CH2 become H2CO3 which decomposes (H20 +
CO2.
H CH3
H CH3 KMnO4
C C H3C C C CH3
CH3 CH3 (warm, conc.) H CH3
OH OH
H3C C + C CH3
O O

OH
H3C C
O
Syn Hydroxylation
of Alkenes
• Alkenes can be converted to a cis-1,2-diols.
• Two reagents:
• Osmium tetroxide (expensive!) - (1. OsO4 2. NaHSO3, H20)

• Cold, dilute aqueous potassium permanganate, H20

CH2CH3
H CH2CH3 (1) OsO4
H C OH
C
(2) H2O2 H C OH
C
H CH2CH3 CH2CH3
cis-3-hexene meso-3,4-hexanediol
 Alkynes
• Alkynes contain a carbon—carbon triple bond.
• Terminal alkynes have the triple bond at the end of the carbon
chain so that a hydrogen atom is directly bonded to a carbon
atom of the triple bond.
• Internal alkynes have a carbon atom bonded to each carbon atom
of the triple bond.
Acidity of Acetylene and Terminal Alkynes
• In general, the C-H bond of hydrocarbons show little tendency to ionize thus
hydrocarbons are very weak acids.
• Relative electronegativities of carbon: sp > sp2 > sp3
• Thus, acetylene and terminal alkynes are stronger acids than other
hydrocarbons (acetylene = pKa ~26).
• A strong base (amide ion) can deprotonate acetylene to generate an acetylide
anion. Terminal alkynes react similarly.
_
_ R C C + B:H
+ B: Na +
R C C H
Na +
Preparation of Alkynes by Alkylation of Acetylene and
Terminal Alkynes
Alkylation of acetylide anions is a general method of making higher
alkynes from simpler alkynes.
Acetylide anions undergo nucleophilic substitution reactions with methyl
halides and primary alkyl halides, resulting in the formation of a C-C
bond. Acetylide anions act as a base with secondary and tertiary alkyl
halides resulting in E2 elimination.

Liq. NH3
H C C H + H2N: Na+ H C C

H C C + H3CH2CH2C-Br H3CH2CH2C C C H
pentyne
 Preparation of Alkynes by Elimination Reactions
Double dehydrohalogenation reaction of dihaloalkanes

H X
X2 R NaNH2,, NH
NH3
R
R
R NaNH
NaNH 22 3
H X
H
vicinal NaNH22, NH3
NaNH
dihalide R
R R R
X
H H
R NaNH
R NaNH22, NH3
X X

geminal
dihalide

3 equivalents of NaNH2 are required for preparing terminal alkynes

from 1,2- or 1,1-dihaloalkanes.
1. NaNH2, NH3
2. H2O
Reactions
Hydrogenation of Alkynes

Metal catalyst = Pt, Pd, Ni or Rh

Hydrogenation of alkynes is a syn addition: cis alkenes are intermediates.

Pd/CaCO3
Lead acetate, quinoline

Metal-Ammonia Reduction of Alkynes

Mechanism
alkenyl radical

alkenyl anion
 Addition of hydrogen halides
• Addition of HX to an alkyne follows Markovnikov’s rule because a secondary
vinylic cation is more stable than a primary vinylic cation.
• When excess hydrogen halide is present, a second equivalent is added
• Markovnikov’s rule also followed for the second addition
• Forms geminal dihalide
Br
H Br
CH3CH2C CH CH3CH2C CH2 CH3CH2C CH2
Br Br
HBr
CH3CH2C CH2 CH3CH2 CCH3

Peroxides have same effect on HBr addition to an alkyne as to an alkene and follows Br

anti-Markovnikov’s rule
 Hydration of Alkynes
• Alkyne hydration employs aq. H2SO4 as the reaction medium and
mercury(II) sulphate as a catalyst to yield an enol (follows MR)
• Formed enol tautomerizes to the keto form

HgSO4
 Addition of halogens to alkynes
• Halogens (Cl2 and Br2) add to alkynes
• Excess halogen leads to the addition of a second equivalent
• Stereochemistry of addition is anti

Cl2 Cl Cl2 Cl Cl
CH3CH2C CCH3 CH3CH2C CCH3 CH3CH2C CCH3
CH2Cl2 Cl CH2Cl2 Cl Cl

Br2 Br Br2 Br Br
CH3C CH CH3C CH CH3C CH
CH2Cl2 Br CH2Cl2 Br Br
 Ozonolysis of Alkynes
Ozonolysis of Alkynes

1) O3
2) Zn O
R C C H + H2CO3
R C
OH 1. O3
terminal alkyne
Carboxylic acid

1) O3
2. H20
O O
2) Zn
R1 C C R2 R1 C + C R2
OH HO
internal alkyne
Carboxylic acids

Alkynes are less reactive toward ozonolysis than alkenes. An alkene can be
oxidatively cleaved by ozone in the presence of an alkyne.