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Chemistry 22: Some Notes on Chapter 10 Reactions

I. Two important themes:A. Opening of neutral and positively charged three-membered ring structures:

1. Always antistereochemistry.2. Neutral three-membered ring structure: nucleophile approaches the less

substiututed carbon.3. Positively charged three-membered ring structure: nucleophile

approaches the more substiututed carbon.

Y

Nuc

Y

Nuc

Nuc

Y Y

Nuc

B. Anti and syn stereochemistry of addition:

Y

X

YX

anti addition

syn addition

Note that anti and syn here describe the stereochemistry of the addition

process, and not necessarily of the product structures.

X

X

X

X

XX

syn

addition

syn

addition

=

anti addition

XX

=

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II. Bromination, chlorination, bromohydrin formation, and chlorohydrinformation (bromonium and chloronium ion intermediates).

(racemic)

Br2

CCl4

CH3Br

Br

(racemic)

Br2

H2O

CH3OH

Br

(racemic)

Br2

ROH

CH3OR

Br

(racemic)

Cl2

CCl4

CH3Cl

Cl

(racemic)

Cl2

H2O

CH3OH

Cl

(racemic)

Cl2

ROH

CH3OR

Cl

Always net anti addition.

III. Oxymercuration-reduction.Reaction accomplishes the same thing as addtion of H3O

+(Markovnikov addition ofH2O or ROH), but without the risk of carbocation rearrangements).

1) Hg(OAc)2 /THF/H2O

2) NaBH4 OH

H3O+

H2OOH

OH

But:

and

1) Hg(OAc)2/THF

CH3CH2OH

2) NaBH4

O

Alkoxymercuration:

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IV. Epoxidation.

CF3CO3H

CF3CO3H

O (racemic)

O

CF3CO3H O

(racemic)

H

O

O

O CF3

Stereochemistry of alkene is retained.mCPBA (meta-chloroperoxybenzoic acid) can also be used in place of CF3CO3H.

V. Opening of epoxides in acid and base.

O H2O

acid or base

OH

OH

(racemic)

O

CH3OH

acid

O

CH3OH

base

OCH3CH3

OH

H

CH3OH

H

OCH3

(racemic)

(racemic)

Starting from an alkene which isepoxidized (previous reaction),the net result is always antiaddition across the double bond.The epoxidation reaction isconcerted and preserves thestereochemistry of the alkene.

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VI. Hydrogenation.A. Reaction: addition of H2across an alkene.

H2

Pd/C or Pt/C

B. A stereospecific syn addition.

and

(racemic)

meso

H2

Pd or Pt

H2

Pd or Pt

H2

Pd or Pt

C. Stereoselectivity: steric effects.

H3C CH3 H3C

CH3

CH3

H

H2

Pd or Pt

D. Hydrogenation of alkynes (C!C to C=C or C-C).

1. H2& Pd catalyst "alkane.

2. H2& Lindlar!s catalyst

"

cisalkene.3. Na/NH3"transalkane.H2

Pd/C (or Pt)

H2

Lindlar's Catalyst

Na/NH3

(or Li/NH3)

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VII. Carbenes: cyclopropanation.

CH2N2(racemic)

CHCl3

KOH

ClCl

CH2I2

Zn(Cu)

(racemic)

Stereochemistry of alkene is preserved.

VII. Ozonolysis.

1) O3

2) [H]

OO

H

1) O3

2) [O]

OO

OH

1) O3

2) [H]

H

O

O [H] = Zn/H2O OR (CH3)2S OR H2/Pd.[O] = H2O2/HOAc.

IX. Hydroxylation: OsO4/NaHSO3.

(racemic)

1) OsO4

2) Na2SO3 H2O

CH3OH

OH

H

The above is asynaddition; contrast to the antiaddition below.

(racemic)

1) CF3CO3H

2) H2O/

acid or base

CH3OH

H

OH

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X. Addition reactions of alkynes are (roughly) analogous to those of alkenes.

RHX

R

X

R

X X

R

X2

R

X

R

X X

X

X

X

R

R

OH

R CH3

H2O/H+

Hg++

O

R

R

R

1) R'2BH

2) H2O2 NaOH

OH

O

H

R R

H2

Lindlar's

Catalyst

Na/NH3R R

RR

R R

(Markovnikov)

(Markovnikov)

(anti-Markovnikov)

H C C H

NaNH2H C C

R XH R

NaNH2

R C CR' X

R R'

R, R' must be 1 (SN2) X = good LG (e.g., OTs, I, Br, Cl)

H

H

H

H

=

=

R R

RR