Elimination Of Hbr From Meso.docx

Elimination of HBr from meso-1,2-dibromo-1,2-diphenylethane gave cis-2bromostilbene, while the (+) or (-) isomer gave the trans alkene.

Anti-elimination has also been demonstrated in cases where the electrofuge is not hydrogen. In the reaction of 2,3-dibromobutane with iodide ion, the two bromines are removed In this case, the meso compound gave the trans alkene and the dl pair the cis

1. No matter what the mechanism, a double bond does not go to a bridge head carbon unless the ring sizes are large enough (Bredt’s rule,). This means, for example, not only that A gives only B and not C. Br

.

C

A

B

2. No matter what the mechanism, if there is a double bond (C=C or C=O) or an aromatic ring already in the molecule that can be in conjugation with the new

double bond, the conjugated product usually predominates, sometimes even when the stereochemistry is unfavorable Predict the product when each of the following undergoes HOFFMEN ELIMINATION. O

Ph

N Me H3C

OH

NMe3

OH

O

Ph

N Me

OH

Give the structure of 3 alkene in decreasing ration

C2H5 H 3C

NMe3

OH





C 2H 5

C2H5

C2H5

Predict the product when each of the following undergoes HOFFMEN ELIMINATION O

Cl

Me

Me2NH LAH/H2O Cl

N Me

Cl

OH O

O

A

MeI EXCESS Ag2O/

B

NMe2

Me2N

NMe2

N Me

B A

If the addition is syn, the product will be the erythro dl pair, because each carbon has a 50% chance of being attacked by Y:

Y

W A

E R Y T H R O d,l P A I R

W Y

A

B

A

B B

A

B

OR

Y Y

W

W

A

A B

A A

B

B

B

On the other hand if the additions is ANTI threo d,l pair will be formed. Y A

A

A

B

B

W B

W

A

Y B

Y

A

A

B

W

A

B

W

A

B

B

T H R E O d,l P A I R

Y

Of course, the trans isomer will give the opposite results: the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition ofBr2), the "erythro pair"is a meso compound. In addition to triple-bond compounds of the type AC≡CA, syn addition results in a cis alkene and anti addition in a trans alkene. Addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective

Catalytic hydrogenation at triple bond is faster than double bond. Halogen addition at triple bond is slower than double bond. H-X addition is slower in triple bond than double bond.

The addition of HCl, HBr, and HI to allene has been studied in some detail. In each case a 2-halopropene is formed, corresponding to protonation at a terminal carbon. The initial product can undergo a second addition, giving rise to 2,2-dihalopropanes. The regiochemistry reflects the donor effect of the halogen. Dimers are also formed, but we have not considered them. X

X

H2C

CH3C=CH2 +

HX

CH2 +

C

CH3CCH3 X

The presence of a phenyl group results in the formation of products from protonation at the center carbon PhHC

C

CH2 +

PhCH=CHCH2Cl

HX

Two alkyl substituents, as in 1,1-dimethylallene, also lead to protonation at the center carbon (H3C)2C

CH2 +

C

HX

(CH3)2C=CHCH2Cl

Isomerisation Acetylene homologues isomerizes when heated with ethanolic potassium hydroxide ,the triple bond moving towards the centre of the chain.

CH3-CH2CCH

KOH

CH3CH=C=CH2

CH3CCCH3

On the other hand when alkynes are heated with sodamide in an inert solvent the triple bond moves towards the end of the chain

H CH3CCCH3 +

NaNH2

NH3

+ CH3CH2CCNa

CH3CH2CCH

Conditions: • Metals: Li, K, Na, occasionally Ca or Mg. • Co-solvents: diethyl ether, THF, glymes. • Proton sources (where appropriate): t-BuOH and EtOH are most common, also MeOH, NH4Cl, and water.