48. Lecture-10
2. Which of the following reaction will go faster if the concentration of the
nucleophile is raised?
(a) I and III (b) I, II and III
(c) l and II (d) none
Can you crack it
49. Lecture-10
Plane Polarised light
Light which is produced by passing ordinary light through a Nicol prism
and which has vibrations in single plane only is called plane polarised
light
Stereo-chemical aspects of nucleophilic substitution reaction
53. If the spatial arrangement of four groups around a central carbon is
tetrahedral and all the substituents attached to that carbon are
different, such a carbon is called asymmetric carbon or stereocenter.
The asymmetry of the molecule is responsible for the optical activity
in such organic compounds.
Asymmetric molecule
55. The objects which are non-superimposable on their mirror image are
said to be chiral and this property is known as chirality.
Chirality
56. A compound having at least one carbon atom having all the four
different groups attached to it is called chiral compound or optically
active compound.
How to identify whether a molecule is chiral?
57. Let us have few examples
★
1. CH3 - CH2 - CH - CH3
|
Br
★
2. CH3 - CH2 - CH - CH = CH2
|
Br
58. Lecture-10
Identify chiral and achiral molecules in each of the following pair of
compounds.
(Wedge and dash representations according to Class XI. Fig 12.1).
60. Lecture-10
5. In which of the following molecules carbon atom marked with an asterisk ()
is asymmetric?
(A) (a), (b), (c), (d)
(C) (b), (c), (d)
(B) (a), (b), (c)
(D) (a), (c). (d)
Can you crack it
63. Lecture-10
50:50 mixture of the above two is obtained then the process is called
racemisation and the product is optically inactive, as one isomer will
rotate light in the direction opposite to another
74. When haloalkanes are heated with alcoholic KOH, they undergo
dehydrohalogenation to form alkenes. These reactions are called 𝛽-
eliminations because the hydrogen atom present at 𝛽- position of the
haloalkane is removed
𝛽
- Elimination Reactions
79. (1) E2 is a concerted mechanism where all the bonds are broken and
formed in a single step. The E1, on the other hand, is a stepwise
mechanism.
(2) E2 reactions are favored by strong bases such as the methoxide
(MeC–), ethoxide (EtO–), potassium tert-butoxide (tBuOK), DBN, DBU,
LDA and etc.
The E1 reactions are favored by weak bases. The most common weak
bases are water and alcohols:
Differences between E1 and E2 mechanisms
80. on the
(3) E2 is a second-order reaction and the rate depends
concentration of both, the substrate and the base.
E2 : Rate = k [Substrate] [Base]
E1 : Rate = k [Substrate]
Differences between E1 and E2 mechanisms
81.
82. Choosing between El and E2 is easier since the reactivity pattern is the
same for both: the rate of El and E2 increase with the degree of
substitution.
El or E2 ? The key factor here is the strength of the base.
E2 reactions are favored by strong bases.
E1 reactions are favored by weak bases.
Weak base – E1 Strong base - E2
Choosing between the El and E2 mechanisms
85. The reactivity of haloalkanes towards elimination reaction follows the
order
Tertiary > Secondary > Primary
𝛽
- Elimination Reactions
86. Among various halides with same alkyl group the order of reactivity is
RI > RBr > RCl
𝛽
- Elimination Reactions
87. In case of two different products, the preferred alkene is the one
which is maximum alkylated i.e., which carries more number of alkyl
groups attached to the doubly bonded carbon atoms. This
generalisation is known as Saytzeff Rule. For example, 2-bromobutane
on reaction with alcoholic KOH gives but-2-enes as the major product
𝛽
- Elimination Reactions
88. Bulky alkyl groups ⇒SN1
Light alkyl groups ⇒SN2
For Nucleophilic substitution
I
Short Tricks
89. When Nucleophile is heavy, it becomes base
II
Aq KOH
CH3 - CH2 - OH
CH3 - CH2 - X
Alc KOH
CH2 = CH2
Short Tricks
91. Match the items of Column I and Column II.
Column I Column II
(i) SN1 reaction (a) vic- dibromides
(ii) Chemicals in fire extinguisher (b) gem- dihalides
(iii) Bromination of alkenes (c) Racemisation
(iv) Alkylidene halides (d) Saytzeff rule
(v) Elimination of HX from alkyl halide (e) Chlorobromocarbons
(A) (i)–c, (ii)–e, (iii)–a, (iv)–b, (v)–d
(B) (i)–a, (ii)–c, (iii)–e, (iv)–b, (v)–d
(C) (i)–d, (ii)–a, (iii)–c, (iv)–b, (v)–e
(D) (i)–e, (ii)–c, (iii)–b, (iv)–a, (v)–d
Can you crack it
92. (A) CH3 - CH = CH2
(B) CH3 - CH - CH3
|
OH
(B) CH3CH2 - CH2 - OH
(D) None
The product of the following reaction is
Alc KOH
CH3 - CH2 - CH2 - Cl ?
Can you crack it
