Lecture No. 04
Resolution of racemic mixtures
Session Objectives
By the end of this session, students will be able to:
➢ Define racemic modification
➢ Discuss factors which lead to formation of racemic modification
➢ Explain various methods for resolving racemic mixtures
www.DuloMix.com 2
Resolution of racemic modifications
• It is a process where by a racemic modification is separated into its two
enantiomers
• It is quantitative and sometimes only one isomer will be isolated
• Several methods have been developed and type of separation will be selected
depending upon the nature of compound
1) Mechanical separation
• Introduced by Pasteur and also known as spontaneous resolution by crytallisation
• Applicable only for the racemic mixtures where the crystal forms of enantiomers
are enantiomorphous – which are separated by hand
www.DuloMix.com
Resolution of racemic modifications
• Pasteur separated sodium ammonium tartrate racemate in this way
• He crystallized sodium ammonium tartrate racemate from a concentrated solution
at room temperature below 28 0C and separated mechanically
2) Preferential crystallization through inoculation
• Supersaturated solution of the racemic modification is treated with a crystal of one
enantiomer, this form is precipitated
• Resolution of glutamic acid by inoculation has been perfectly suitable for industrial
use
• But this method was found impractical with amino acids
www.DuloMix.com
Resolution of racemic modifications
3) Biochemical separation
• Certain bacteria and moulds when they grow in dilute solution of racemic
modification, destroy one enantiomer
• For example, Pencillium glaucum when grown in solution of racemic ammonium
tartrate, attacks the (+)-form and leaves (-)-form
Drawbacks
• Dilute solutions must be used, so amounts obtained will be small
• One form is always destroyed and other form is not always obtained in 50%, some
may also be destroyed
• To find a microorganism which will attack only one enantiomer
www.DuloMix.com
Resolution of racemic modifications
4) Conversion into diastereoisomers
• Best of all methods of resolution
• Racemic modification is treated with optically active substance
• Diastereoisomers produced are separated by fractional crystallization
• For example, racemic acids can be separated by optically active bases
(Dacid + Lacid) + 2Dbase (Dacid Dbase) + (Lacid Dbase)
• Now the two diastereoisomers can be separated by fractional crystallization
www.DuloMix.com
Resolution of racemic modifications
• Enantiomers of acids can be regenerated by hydrolysis with inorganic acids or with
alkalis
• Further purification can be done by chromatography
• For resolution, compounds used are-
• Acids: optically active bases are alkaloids- brucine, quinine, strychnine, morphine,
cinchonine
• Bases: optically active acids used are tartaric acid, camphor-β-sulphonic acid
• Alcohols: converted to acid ester using succinic or phthalic anhydride
• Aldehydes and ketones: by means of optically active hydrazines
• Amino compounds: by means of optically active aldehydes
www.DuloMix.com
Resolution of racemic modifications
5) Chromatography
• Optically active substances may be selectively adsorbed by some optically active
adsorbent
• For example, Henderson and Rule (1939) partially resolved p-Phenylene bisimino
camphor on lactose as adsorbent
• Bradley and Easty (1951) have found that wool and casein selectively adsorb (+)-
mandelic acid from an aqueous solution of (±)-mandelic acid
• Separation of quinine and quinidine, cinchonine and cinchonidine by passing
through silica gel
www.DuloMix.com
Resolution of racemic modifications
6) Resolution through formation of Molecular Complexes
• Common method of resolution because of weak interactions between substrate
and resolving agent
• Other methods involves formation of stable salts or covalent compounds
• Complexes should be easy to form and easy to decompose
• For example, complexation with platinum reagents has been used to resolve
trans-cyclooctane
www.DuloMix.com
Resolution of racemic modifications
7) Biochemical methods of resolution using enzymes
• Laboratory reduction of 2-butanone gives racemic product of 2-butanol
• But reduction in the presence of chiral enzyme gives optically pure (R)-2-butanol
• Easy to acetylate racemic alcohol and treat racemic esters with lipase
• One enantiomer is hydrolyzed to alcohol while other remains as ester
• Further separated by chromatography
• Enzyme acyalse 1 (hog kidney acylase) is capable of hydrolyzing amide links (-NHAc)
of L-amino acids only (stereospecificity)
www.DuloMix.com
Resolution of racemic modifications
8) Resolution via chiral recognition and Inclusion compounds
• Chiral recognition- only one enantiomer fits into chiral host activity, while the other
does not
• Generally both diastereoisomers may be formed, one is formed more readily than
other
• For example, aqueous solution of rac-amine salt is mixed with chloroform solution of
optically active crown ether, could separate complex of chiral crown ether and (R)
amine salt
• Cyclodextrins (CDs) are made up of six, seven or eight glucose units connected in a
large ring termed as α, β and γ CDs
• CDs are of tub shape with primary OH groups projecting from narrow side and
secondary OH groups from wide side
• Can be used for resolution of variety of compounds via inclusion complexes
www.DuloMix.com
Structure of cyclodextrins
www.DuloMix.com
Structures of common crown ethers
www.DuloMix.com
Summary
• Racemic modification is separation of its isomers
• Supersaturated solution of the racemic modification is treated with a crystal of
one enantiomer, this form is precipitated
• Racemic modification is treated with optically active substance is the best
method
• Optically active substances may be selectively adsorbed by some optically active
adsorbent
www.DuloMix.com 14
Thank You
www.DuloMix.com