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Reactions of Alkenes: Addition Reactions, Schemes and Mind Maps of Organic Chemistry

Randolph CollegeOrganic Chemistry

Organic Chemistry chapter 6 on the Reactions of Alkenes: Addition Reactions

Typology: Schemes and Mind Maps

2020/2021

Uploaded on 03/30/2021

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Chapter 6: Reactions of Alkenes: Addition Reactions
6.1: Hydrogenation of Alkenes – addition of H-H (H2) to the
π-bond of alkenes to afford an alkane. The reaction must be
catalyzed by metals such as Pd, Pt, Rh, and Ni.
C C
H
H H
H
H H
+C C
H
H H
H
HH
Pd/C
EtOH
ΔH°hydrogenation = -136 KJ/mol
C-C π-bond H-H C-H
= 243 KJ/mol = 435 KJ/mol = 2 x -410 KJ/mol = -142 KJ/mol
• The catalysts is not soluble in the reaction media, thus this
process is referred to as a heterogenous catalysis.
• The catalyst assists in breaking the π-bond of the alkene and
the H-H σ-bond.
• The reaction takes places on the surface of the catalyst. Thus,
the rate of the reaction is proportional to the surface area
of the catalyst.
128128
H2, PtO2
ethanol
OO
OCH3
O
H2, Pd/C
ethanol
OCH3
O
CN
CN
H2, Pd/C
ethanol
C5H11 OH
O
Linoleic Acid (unsaturated fatty acid)
H2, Pd/C
CH3(CH2)16CO2H
Steric Acid (saturated fatty acid)
• Carbon-carbon π-bond of alkenes and alkynes can be reduced
to the corresponding saturated C-C bond. Other π-bond bond
such as C=O (carbonyl) and C≡N are not easily reduced by
catalytic hydrogenation. The C=C bonds of aryl rings are not
easily reduced.
pf3
pf4
pf5
pf8
pf9
pfa
pfd

Partial preview of the text

Download Reactions of Alkenes: Addition Reactions and more Schemes and Mind Maps Organic Chemistry in PDF only on Docsity!

Chapter 6: Reactions of Alkenes: Addition Reactions

6.1: Hydrogenation of Alkenes – addition of H-H (H

) to the

π-bond of alkenes to afford an alkane. The reaction must be

catalyzed by metals such as Pd, Pt, Rh, and Ni.

C C

H

H H

H

H H

C C

H

H H

H

H

H

Pd/C

EtOH

ΔH°

hydrogenation

= -136 KJ/mol

C-C π-bond H-H C-H

= 243 KJ/mol = 435 KJ/mol = 2 x -410 KJ/mol = -142 KJ/mol

• The catalysts is not soluble in the reaction media, thus this

process is referred to as a heterogenous catalysis.

• The catalyst assists in breaking the π-bond of the alkene and

the H-H σ-bond.

• The reaction takes places on the surface of the catalyst. Thus,

the rate of the reaction is proportional to the surface area

of the catalyst.

H

2

, PtO 2

ethanol

O

O

OCH

3

O

H

2 , Pd/C

ethanol

OCH

3

O

C

N

C

N

H

2 , Pd/C

ethanol

C

5

H

11

OH

O

Linoleic Acid (unsaturated fatty acid)

H

2 , Pd/C

CH

3

(CH

2

16

CO

2

H

Steric Acid (saturated fatty acid)

• Carbon-carbon π-bond of alkenes and alkynes can be reduced

to the corresponding saturated C-C bond. Other π-bond bond

such as C=O (carbonyl) and C≡N are not easily reduced by

catalytic hydrogenation. The C=C bonds of aryl rings are not

easily reduced.

6.2: Heats of Hydrogenation - an be used to measure relative

stability of isomeric alkenes

H

3

C CH

3

H H

H

3

C H

H CH

3

cis- 2 - butene trans- 2 - butene

ΔH°

combustion

: -2710 KJ/mol -2707 KJ/mol

H

3

C CH

3

H H

H

3

C H

H CH

3

cis- 2 - butene trans- 2 - butene

H

2

, Pd H 2 , Pd

CH

3

CH

2

CH

2

CH

3

ΔH°

hydrogenation

: -119 KJ/mol -115 KJ/mol

trans isomer is ~4 KJ/mol more stable than the cis isomer

trans isomer is ~3 KJ/mol

more stable than the

cis isomer

The greater release

of heat, the less

stable the reactant.

H 3 C CH 3

H H

H 3

C H

H CH 3

H 3

C H

H H

H 3

C H

H 3 C H

H 3

C CH 3

H 3 C H

H 3 C CH 3

H 3 C CH 3

tetrasubstituted

trisubstituted

disubstituted

monosubstituted

H 2 C=CH 2

Alkene

! H° (KJ/mol)

Table 6.1 (pg 228): Heats of Hydrogenation of Some Alkenes

6.5: Regioselectivity of Hydrogen Halide Addition:

Markovnikov's Rule

Reactivity of HX correlates with acidity:

slowest HF << HCl < HBr < HI fastest

For the electrophilic addition of HX across a C=C bond, the H (of

HX) will add to the carbon of the double bond with the most H’s

(the least substitutent carbon) and the X will add to the carbon of

the double bond that has the most alkyl groups.

R C

C H

H

H

R C

C H

R

H

R C

C H

R

R

H-Br

H-Br

H-Br

C C

Br

H

R

H

H

H C C

H

H

R

Br

H

H

C C

Br

R

R

H

H

H C C

H

R

R

Br

H

H

C C

Br

R

R

H

H

R C C

H

R

R

Br

H

R

none of this

H C

C H

R

R'

H-Br

C C

Br

H

R

H

H

R C C

H

H

R

Br

H

R'

Both products observed

none of this

none of this

Mechanism of electrophilic addition of HX to alkenes

6.6: Mechanistic Basis for Markovnikov's Rule:

Markovnikov’s rule can be explained by comparing the

stability of the intermediate carbocations

For the electrophilic addition of HX to an unsymmetrically

substituted alkene:

• The more highly substituted carbocation intermediate is

formed.

• More highly substituted carbocations are more stable than

less substituted carbocations. (hyperconjugation)

• The more highly substituted carbocation is formed faster

than the less substituted carbocation. Once formed, the

more highly substituted carbocation goes on to the final

product more rapidly as well.

6.7: Carbocation Rearrangements in Hydrogen Halide

Addition to Alkenes - In reactions involving carbocation

intermediates, the carbocation may sometimes rearrange if a

more stable carbocation can be formed by the rearrangement.

These involve hydride and methyl shifts..

C C

C

H

3

C

H

3

C

H

H

H

H

H-Cl

C C

C

H

3

C

H

3

C

H

H

H

H

Cl

H

C C

C

H

3

C

H

3

C

Cl

H

H

H

H

H

expected product

Note that the shifting atom or group moves with its electron pair.

A MORE STABLE CARBOCATION IS FORMED.

C C

C

H

3

C

H

3

C

CH

3

H

H

H

H-Cl

C C

C

H

3

C

H

3

C

CH

3

H

H

H

Cl

H

C C

C

H

3

C

H

3

C

Cl

H

H

H

H

3

C

H

6.9: Addition of Sulfuric Acid to Alkenes (please read)

6.10: Acid-Catalyzed Hydration of Alkenes - addition of water

(H-OH) across the π-bond of an alkene to give an alcohol;

opposite of dehydration

C CH

2

H

3

C

H

3

C

H

2

SO

4

, H

2

O

H

3

C

C OH

H

3

C

H

3

C

This addition reaction follows Markovnikov’s rule The more

highly substituted alcohol is the product and is derived from

The most stable carbocation intermediate.

Reactions works best for the preparation of 3° alcohols

Mechanism is the reverse of the acid-catalyzed dehydration

of alcohols:

Principle of Microscopic Reversibility

Principle of Microscopic Reversibility

6.11: Thermodynamics of Addition-Elimination Equlibria

6.11: Thermodynamics of Addition-Elimination Equlibria

C OH

H

3

C

H

3

C

H

3

C

H

3

C

C

H

3

C

CH

2

+ H

2

O

H

2

SO

4

How is the position of the equilibrium controlled?

Le Chatelier’s Principle - an equilibrium will adjusts to any stress

The hydration-dehydration

The hydration-dehydration

equilibria

equilibria

is pushed toward

is pushed toward

hydration

hydration

(alcohol) by adding water

(alcohol) by adding water

and toward

and toward

alkene

alkene

(dehydration) by

(dehydration) by

removing waterremoving water

Bonds broken Bonds formed

C=C π-bond 243 KJ/mol H

C-H

C – H -410 KJ/mol

H – OH 497 KJ/mol (H

C)

C – OH -380 KJ/mol

calc. ΔH° = -50 KJ/mol

ΔG° = -5.4 KJ/mol ΔH° = -52.7 KJ/mol ΔS° = -0.16 KJ/mol

The acid catalyzed hydration is not a good or general method for

the hydration of an alkene.

Oxymercuration : a general (2-step) method for the Markovnokov

hydration of alkenes

H

3

C O

C

O

Ac= acetate =

NaBH

reduces the C-Hg

bond to a C-H bond

C

4

H

9

C

C

1 ) Hg(OAc) 2

, H

2

O

C

4

H

9

C

C

H

Hg(OAc)

H

H OH

H

H

H

2 ) NaB H 4

C

4

H

9

C

C

H

H

H

H OH

6.17: Mechanism of Halogen Addition to Alkenes:

Halonium Ions - Bromonium ion intermediate explains the

stereochemistry of Br

addition

6.18: Conversion of Alkenes to Vicinal Halohydrins

C C

C C

X OH

halohydrin

alkene

"X-OH"

X

OH

anti

stereochemistry

X

2

, H

2

O

+ HX

Mechanism involves a halonium ion intermediate

For unsymmterical alkenes, halohydrin formation is

Markovnikov-like in that the orientation of the addition of

X-OH can be predicted by considering carbocation stability

more δ

charge on the

more substituted carbon

Br adds to the double bond first (formation of

bromonium ion) and is on the least substituted

end of the double bond

H

O adds in the second step and adds to the

carbon that has the most δ

charge and ends

up on the more substituted end of the double bond

CH

3

Br

CH

CH

HO

Br

H

Br

, H

O

+ HBr

Organic molecules are sparingly soluble in water as solvent. The

reaction is often done in a mix of organic solvent and water using

N-bromosuccinimide (NBS) as he electrophilic bromine source.

DMSO, H

2

O

N

O

O

  • Br

Br

OH

N

O

O

H

Note that the aryl ring does not react!!!

6.19: Epoxidation of Alkenes - Epoxide (oxirane) : three-

membered ring, cyclic ethers.

Reaction of an alkene with a peroxyacid:

peroxyacetic acid

H

3

C

O

O

O

H

H

3

C

O

OH

peroxyacetic

acid

acetic

acid

HO OH

peroxide

H

3

C

O

O

O

H

O

H

3

C

OH

O

6.21: Introduction to Organic Chemical Synthesis

Synthesis: making larger, more complex molecules out of

less complex ones using known and reliable reactions.

devise a synthetic plan by working the problem backward from

devise a synthetic plan by working the problem backward from

the target molecule

the target molecule

OH ??

H 2 SO 4 H 2 , Pd/C

??

OH

6.22: Reactions of Alkenes with Alkenes: Polymerization

(please read)

??

CH Br 3

CH 3

Br

H