1 electrophilic reactions. 2 3 electrophilic reactions addition to unsaturated carbon alkenes +...

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Electrophilic Reactions

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Electrophilic Reactions

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Electrophilic ReactionsAddition to unsaturated carbon

Alkenes + Alkynes

Unsaturated hydrocarbonUnsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds

- alkenealkene: contains a carbon-carbon double bond and has the general formula CnH2n

- alkyne- alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2

Ethene(an alkene)

H

C C

H

H H

H-C C-HEthyne

(an alkyne)

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Alkenes

IUPAC Nomenclature

-use the infix -enen- to show the presence of a carbon-carbon double bondnumber the parent chain to give the 1st carbon of the double bond the lower numberfollow IUPAC rules for numbering and naming substituents- for a cycloalkene, the double bond must be numbered 1,2

1-Hexene 4-Methyl-1-hexene2-Ethyl-3-methyl-

1-pentene

1 1

12 2 2

3 3

34 4 4

5 5

56 6

1 2

34

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3-Methylcyclo-pentene

CH3

1,6-Dimethylcyclo-hexene

CH3

CH31

65

4

3

2

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Alkenes

Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

CH2=CH2 CH3CH=CH2 CH3C=CH2

CH3

IUPAC:IsobutylenePropyleneEthyleneCommon:

2-MethylpropenePropeneEthene

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Alkenes

The cis,trans system:The cis,trans system: configuration is determined by the orientation of atoms of the main chain

cis-3,4-Dimethyl-2-pentene

1

2 3

4C

H

C

CH3

CH(CH3)2H3C

trans-3-Hexene

C

H

C

CH2CH3

HCH3 CH2

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Alkenes

Configuration: E,Z

Assign a priority to the substituents on each carbon of the double bondif the groups of higher priority are on the same side of the double bond, the configuration is ZZ (German: zusammen, together)

if the groups of higher priority are on opposite sides of the double bond, the configuration is EE (German: entgegen, opposite)

Z (zusammen) E (entgegen)

C

higher

C

higher

lowerlower

C

lower higher

C

lowerhigher

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Alkenes – E,Z configuration

Priority rules1. Priority is based on atomic number; the higher the atomic number, the higher the

priority

2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference

3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds

(53)(35)(17)(16)(8)(7)(6)(1)

Increasing priority

-H -CH3 -NH2 -OH -SH -Cl -Br -I

Increasing priority

(8)(7)(6)(1)-CH2-OH-CH2-NH2-CH2-CH3-CH2-H

-CH=CH2

O

-CH

O

H

C

C

O

CC

-CH-CH2is treated as

is treated as

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Alkenes –Addition of hydrogen halides

Alkenes –Addition of water

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Alkenes –Addition of hydrogen halides to double bonds with substituted carbons

Carbocations are planar -> attack from both sides possible!!

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Alkenes –Addition of hydrogen halides to double bonds with substituted carbons-> Addition is regioselective (regioselective reaction:regioselective reaction: a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions)

Markovnikov’s rule:Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens

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Alkenes –Addition of hydrogen halides to double bonds with substituted carbons

-> Carbocation rearrangements

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Alkenes –Addition of halogens

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-> Stereochemical consequences of electrophil addition

-> gives 2,3 dibromobutane as a pair of enantiomers, R,R and S,S

-> a result of anti addition

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-> gives 2,3 dibromobutane as a meso R,S isomer

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-> gives 1,2 dibromocyclohexan as a pair of enantiomers, R,R and S,S

-> a result of anti addition

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Alkenes –Addition of halogens in water -> Halo alcohol (halohydrin)

Halo alcohols can be used to make -> epoxides

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Alkenes –Addition of halogens in water under basic conditions -> Epoxides

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Alkynes

IUPAC nomenclatureuse the infix -ynyn- to show the presence of a carbon-carbon triple bondnumber the parent chain to give the 1st carbon of the triple bond the lower numberfollow IUPAC rules for numbering and naming substituents

3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne

1 12 2

3

3 44 5

6 7

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Alkynes –Addition of halides

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Alkynes –Addition of halides

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Alkynes – Hydration gives a keton

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Electrophilic ReactionsAddition to conjugated systems

Alkenes – conjugated systems

Dienes, trienes, and polyenesfor an alkene with nn carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n2n cis-trans isomers are possibleconsider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here

C2-C3 C4-C5

Double bond

trans transtrans ciscis transcis cis

trans,trans-2,4-heptadiene

trans,cis-2,4-heptadiene

2 24 4

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Alkenes – conjugated systems

- vitamin A has five double bonds - vitamin A is the all-trans isomer

Vitamin A aldehyde (retinal)

enzyme-catalyzedoxidation

H

OVitamin A (retinol)

OH

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Alkenes – addition of halides

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Alkenes – addition of halides

Conjugate addition

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Alkenes – addition of halogens

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Electrophilic ReactionsCarbocations as electrophiles

Cationic polymerization

Polystyrene

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Cationic polymerization – intramolecular electrophilic addition

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Cationic polymerization – intramolecular electrophilic additionTerpenoids and SteroidsTerpenoids and Steroids

Huge group of natural products -> flavouring, aromatherapy oils, Huge group of natural products -> flavouring, aromatherapy oils, vitamins (A, K), steroid hormons (lanosterol, cholesterol)vitamins (A, K), steroid hormons (lanosterol, cholesterol)

Terpene:Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

2-Methyl-1,3-butadiene (Isoprene)

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34head tail

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– myrcene, C10H16, a component of bayberry wax and oils of bay and verbena

– menthol, from peppermint

– Vitamin A

OH

OH

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Lemon oil Pine oil Eucalyptus oil

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Catalytic HydrationAddition of hydrogen to alkenes and alkynes

Catalytic hydration -> NOT a radical reaction (Chapter 9: Radical reactions)

Involvement of atomic hydrogen -> mechanism has more in common with radical reaction than with ionic reaction

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Catalytic HydrationAddition of hydrogen to alkenes and alkynes

1 electrophilic reactions. 2 3 electrophilic reactions addition to unsaturated carbon alkenes +...

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carbon carbon double

carboncarbon triple

unsaturated carbon alkenes

double bond z

double bonds

opposite slide

direction of bond

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