Haloalkanes and Haloarenes 4 Flashcards
what are optically active compounds
Plane of plane polarised light produced by
passing ordinary light through Nicol prism is rotated when it
is passed through the solutions of certain compounds. Such
compounds are called optically active compounds.
Nicol prism is called polariser. it is made up of calcite
what is a polarmeter
The angle
by which the plane polarised light is rotated is measured by an
instrument called polarimeter
what is
i) dextrorotatory
ii)laevorotatory
If the compound rotates the plane
of plane polarised light to the right, i.e., clockwise direction, it
is called dextrorotatory (Greek for right rotating) or the d-form
and is indicated by placing a positive (+) sign before the degree
of rotation.
If the light is rotated towards left (anticlockwise
direction), the compound is said to be laevo-rotatory or the
l-form and a negative (–) sign is placed before the degree of
rotation.
Such (+) and (–) isomers of a compound are called
optical isomers and the phenomenon is termed as optical
isomerism
what is chiral carbon
If all substituents attached to a carbon are different, then the carbon is said to be chiral or asymmetric or stereocentre.
The resulting molecule would lack symmetry( asymmetric or chiral molecule)
The asymmetry of the molecule along
with non superimposability of mirror images is responsible for
the optical activity in such organic compounds.
2-chlorobutane, 2, 3-dihyroxypropanal, (OHC–CHOH–CH2OH),
bromochloro-iodomethane (BrClCHI), 2-bromopropanoic acid
(H3C–CHBr–COOH), etc
what are enantiomers
The stereoisomers related to each other as nonsuperimposable mirror images are called enantiomers
Enantiomers possess identical physical properties namely,
melting point, boiling point, refractive index, etc. They only differ
with respect to the rotation of plane polarised light. If one of the
enantiomer is dextro rotatory, the other will be laevo rotatory.
what is racemic mixture or racemic modification
A mixture containing two enantiomers in equal proportions
will have zero optical rotation, as the rotation due to one isomer
will be cancelled by the rotation due to the other isomer. Such
a mixture is known as racemic mixture or racemic
modification. A racemic mixture is represented by prefixing dl
or (±) before the name, for example (±) butan-2-ol.
what is racemisation
The process
of conversion of enantiomer into a racemic mixture is known as
racemisation.
what is retention of configuration
Retention of configuration is the preservation of the spatial
arrangement of bonds to an asymmetric centre during a chemical
reaction or transformation.
In general, if during a reaction, no bond to the stereocentre is broken,
the product will have the same general configuration of groups
around the stereocentre as that of reactant. Such a reaction is said
to proceed with retention of the configuration.
give an example of retention of configuration? why is the optical activity different?
the reaction that takes place when (–)-2-methylbutan-1-ol
is heated with concentrated hydrochloric acid.
It is important to note that configuration at a symmetric centre in
the reactant and product is same but the sign of optical rotation
has changed in the product. This is so because two different
compounds with same configuration at asymmetric centre may have
different optical rotation. One may be dextrorotatory (plus sign of
optical rotation) while other may be laevorotatory (negative sign of
optical rotation).
what are the 3 general outcomes of a reaction with a chiral carbon
There are three outcomes
for a reaction at an asymmetric carbon atom, when a bond directly
linked to an asymmetric carbon atom is broken.
If (A) is the only compound obtained, the process is called retention
of configuration. Note that configuration has been rotated in A.
If (B) is the only compound obtained, the process is called inversion
of configuration. Configuration has been inverted in B.
If a 50:50 mixture of A and B is obtained then the process is called
racemisation and the product is optically inactive, as one isomer will
rotate the plane polarised light in the direction opposite to another.
in which reaction does inversion take place
In case of optically active alkyl halides, the product formed as a
result of SN2 mechanism has the inverted configuration as compared
to the reactant. This is because the nucleophile attaches itself on the
side opposite to the one where the halogen atom is present. When
(–)-2-bromooctane is allowed to react with sodium hydroxide,
(+)-octan-2-ol is formed with the –OH group occupying the position
opposite to what bromide had occupied.
Thus, SN
2 reactions of optically active halides are accompanied by
inversion of configuration.
in which reaction does racemisation take place
In case of optically active alkyl halides, SN1 reactions are
accompanied by racemisation. Can you think of the reason why it
happens? Actually the carbocation formed in the slow step being sp
2
hybridised is planar (achiral). The attack of the nucleophile may be
accomplished from either side of the plane of carbocation resulting in
a mixture of products, one having the same configuration (the –OH attaching on the same position as halide ion) and the other having
opposite configuration (the –OH attaching on the side opposite to halide
ion). This may be illustrated by hydrolysis of optically active
2-bromobutane, which results in the formation of (±)-butan-2-ol.
what is beta elimination
When a haloalkane with b-hydrogen atom is heated with alcoholic
solution of potassium hydroxide, there is elimination of hydrogen
atom from b-carbon and a halogen atom from the a-carbon atom.
As a result, an alkene is formed as a product. Since b-hydrogen
atom is involved in elimination, it is often called b-elimination.
what it saytseff rule or zaitsev rule
“in dehydrohalogenation reactions, the preferred product is that
alkene which has the greater number of alkyl groups attached to the
doubly bonded carbon atoms.
2 bromopentane can give
1 pentene (19%)
2 pentene (81%)
when approached with a base: does elimination or substitution takes place? wha factors influence it?
An
alkyl halide with b-hydrogen atoms when reacted with a base or a nucleophile has two
competing routes: substitution (SN
1 and SN
2) and elimination. Which route will be taken
up depends upon the nature of alkyl halide, strength and size of base/nucleophile and
reaction conditions. Thus, a bulkier nucleophile will prefer to act as a base and abstracts
a proton rather than approach a tetravalent carbon atom (steric reasons) and vice versa.
Similarly, a primary alkyl halide will prefer a SN
2 reaction, a secondary halide- SN
2 or
elimination depending upon the strength of base/nucleophile and a tertiary halide- SN
1 or
elimination depending upon the stability of carbocation or the more substituted alkene
Elimination:
(CH3)3CO- + (CH3)2CHBr ——> CH3-CH=CH2 + (CH3)3OH
Substitution
OH + (CH3)2CHBr ———>(CH3)2CHOH + Br-
