MCAT Study Guide Ochem Ch. 5 – Alkenes and Alkynes 2017-08-15T06:45:06+00:00

I.          5.1:  REACTIONS OF ALKENES AND ALKYNES

A.     ELECTROPHILIC ADDITION REACTIONS

π bond is broken and two σ bonds in the product result;  π electrons are more loosely held than σ electrons and can be nucleophilic

1.     MARKOVNIKOV ADDITIONS

a)     H—X addition across a bond

(1)   Markovnikov’s addition  (most stable  carbocation)

b)     Acid-Catalyzed Hydration of Alkenes – alkene is added to acid/water (usually sulfuric acid); ends with an alcohol

c)     Oxymercuration- Demercuration ㅡ Markovnikov alcohol is formed, but no carbocation rearrangement!!!

 2.     ANTI-MARKOVNIKOV ADDITIONS

a)     Hydroboration ㅡ synthesis of alcohols where the -OH attaches to the least substituted of the 2 carbons

(1)   Don’t worry about mechanism!!

b)     HBr and RㅡOㅡOㅡR ㅡ Antimarkovnikov!!

(1)   Radicals form

(2)   Carbon radical intermediate is 3° (most stable)

(3)   Only works with H-Br

c)     Addition of Halogens

[X2] to a π Bond – easy, no picture

(1)   Dihalogen breaks π bond with the result of a halogen σ bond on each side

d)     Mechanism of Dihaloalkane Formation

(1)   Addition of bromine bridge makes the attached Cs slightly positive, more electrophilic

(2)   Bromine ions can attack from either side, giving racemic mixture of enantiomers

(3)   If bromine is in water, instead of 2 Br constituents, there will be a Br and OH

e)     Epoxide Formation and Hydrolysis

(1)   Only thing to know is that when an alkene reacts with a peroxy acid, an epoxide is formed

Peroxy acid

(2)   Also note, when an epoxide is hydrolyzed under acidic or basic conditions, anti addition (trans formation) is formed

Epoxide

3.     OXIDATION OF Π BONDS WITH DILUTE KMnO4 OR OsO4 ㅡ cis-diols are formed (syn-addition → 2 OH’s on one side of the π bond

4.     HYDROGENATION ㅡ unsaturated hydrocarbons can be reduced by H2 in presence of metal catalyst

a)     Hydrogenation of π Bonds (syn-addition of H-H) ㅡ easy

(1)   H2 in presence of Ni, Pd, or Pt will break π bond; end product is cis structure

b)     Hydrogenation of π Bonds of Alkynes ㅡ can fully hydrogenate, or can stop at alkene

(1)   2H2 with metal and CaCO3 or BaSO4

(a)   End product is alkene with added H’s in cis formation

(i)          Calcium and barium sound like “cis”ters

(b)   Lindlar catalyst is used here

(2)   2H2 with sodium metal and liquid ammonia

(a)   End product is alkene with H’s in trans formation

(i)          Transvestites need ot use ammonia to remove their nail polish

5.     Ozonolysis:  ozone cleaves double bond resulting in aldehyde or ketone products

 

II.          5.2:  AROMATIC COMPOUNDS

A.     Criteria:

1.     All Cs are sp2 (planar, 3 groups)

2.     Cyclic

3.     Must have Huckel number of π electrons

a)     4n +2 (2, 6, 10, 14, etc…)

b)     Remember resonance structures; these electrons are included in #!

B.     Nomenclature of Substituted Benzenes

1.     Benzoic acid ― benzene with a carboxylic acid functional group

a)     In relation to the primary functional group, other groups are in these positions:

(1)   ortho (o) ― carbon next to the functional group

(2)   meta (m) ― 2 carbons away from the functional group

(3)   para (p) ― on the carbon opposite from the functional group

C.    Reactivity of Aromatic Coumpounds:  Electrophilic Aromatic Substitution

1.     Benzenes are typically very stable and will only react with very electrophilic reagents and will then undergo overall substitution reaction for an H on the ring

2.     When a group is already present on the ring, it can affect the electrophilic aromatic substitution in 2 ways:

a)     The present group will either make it easier (ring activating) or harder (ring deactivating) to introduce the new group (electron-donating groups are activating, electron-withdrawing groups are deactivating

b)     Electron donators (activating) tend to add other groups at ortho/para, include:

(1)   ―NR2

(2)   ―OH

(3)   ―OR

(4)   ―NHC=OR

(5)   ―OC=OR

(6)   these decrease acidity!!!

c)     Electron withdrawers (deactivating) tend to add other groups in meta position, include:

(1)   ―NR3+

(2)   ―NO2

(3)   ―C☰N

(4)   ―SO3H

(5)   ―C=OR

(6)   ―C=OOH

(7)   ―C=OOR

(8)   ―C=ONH2

(9)   ―NH3+

(10) ―Cl, Br, I (mild deactivators)

(11) These increase acidity!!!

III.          5.3: SUMMARY OF REACTIONS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 5 SUMMARY:

○       The loosely held electrons in C⚌C π bonds can act as nucleophiles in addition reactions, which replace one π bond with 2 σ bonds

○       When the π electrons attack electrophiles, the resulting carbocation will be on the more substituted carbon and yield the Markovnikov (more substituted) product

○       Addition reactions that put the new, non-H group on the less substituted C are termed anti-Markovnikov additions

○       Ant-addition puts two new substituents on opposite sides of the planar double bond, while syn-addition puts two new substituents on the same side of the planar double bond

○       Addition reactions are usually not stereospecific since the alkene is planar. Electrophiles will add with equal frequency to both faces of the bond, giving mixtures of enantiomers

○       Double bonds are split by ozonolysis, yielding two carbonyl-bearing compounds

○       Alkenes and alkynes can be hydrogenated by use of H2 and metal catalysts

○       Aromatic compounds are cyclic, planar, conjugated (sp2), have 4n +2 π electrons, and are exceptionally stable

○       Under the influence of very strong electrophiles, aromatic electrons can be nucleophilic, resulting in aromatic substitution

○       Substituents that add electron density to a benzene ring are activating for substitution chemistry, and favor ortho and para positions

○       Sutstituents that withdraw electron density from benzene ring are deactivating, and all but the halogens favor substitutions at meta positions

MCAT Study Guide Ochem - Kim Matsumoto


More MCAT Study Guide Ochem

1.

Ch. 2 Basics of Organic Chemistry

2.

Ch. 3 Bonds + Isomers

3.

Ch. 4 Alkane Reactions

4.

Ch. 5 Alkenes and Alkynes

5.

Ch. 6 Aldehydes and Ketones

6.

Ch. 7 Lab Techniques

7.

Ch. 8 Amino Acids + Carbohydrates

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