Enthalpy

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Alkenes
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Session objectives

1. Kolbes method
2. Dehydration of alcohol
3. Dehydrohalogenation
4. Miscellaneous method

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General characteristics of alkene

• Olefins
• General molecular formula CnH2n
• CC bond hybridization 1.34 A0
• sp2 hybridization
• Show chain, positional and geometrical isomerism

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General characteristics of alkene
Chain isomerism
Positional isomerism
1-pentene
2-pentene
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General characteristics of alkene
Geometric isomerism
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Preparation of alkenes
The Kolbes electrolysis
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Dehydration of alcohols
(E1 - elimination)
1-butene
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Mechanism
Oxonium ion
Carbocation
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Features of dehydration

• A carbocation is formed as an intermediate.

• The ease of dehydration of alcohols is 3 gt 2 gt
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• 3 carbocation is most stable

gt
gt
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Features of dehydration
The stability of alkenes is governed by
Saytzeffs rule
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Features of dehydration
Dehydrohalogenation of alkyl halides (E2 or
1,2-elimination or -elimination)
Mechanism
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Features of dehydration

• Stereochemically, the best conformation for
  elimination is the anti-coplanar conformation,
  i.e. the three bonds HC, CC and CBr are in one
  plane.

• There is no formation of any intermediate in this
  reaction, rather it proceeds through a
  transition state.

• A trans- or anti- elimination in which the atoms
  leave from the opposite sides is observed in most
  cases

• The reaction is thus stereospecific, since a base
  pulls a hydrogen, a strong base will accelerate
  E2 elimination.

• predominant formation of a substituted alkene is
  formed according to Saytzeffs rule

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Features of dehydration
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Hoffmans elimination
If the leaving group is very strong
electron-attracting, then elimination takes place
contrary to Saytzeffs rule. The formation of a
less substituted alkene is observed.
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Hoffmans elimination
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Miscellaneous Methods
Debromination of vicinal dibromides
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Miscellaneous Methods
Corey-House method
Wittig reaction
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Miscellaneous Methods
Catalytic dehydrogenation
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Class Exercise - 1
Solution
Wittig reaction.
Hence, the answer is (b).
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Class Exercise - 2
Solution
CH2 CH CH CH2
Hence, the answer is (c).
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Class Exercise - 3
Solution
sp2, sp3 from the structure.
Hence, the answer is (a).
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Class Exercise - 4
When an aqueous solution of sodium acetate and
sodium propionate is electrolyzed, one gets (a)
ethane (b) propane (c) butane (d) All of
these
Solution
Along with normal butane we get unwanted product
like ethane, propane, etc.
Hence, the answer is (d).
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Class Exercise - 5
Which has the least heat of hydrogenation? (a)
1, 3-butadiene (b) 1-butene (c)
Trans-2-butene (d) Cis-2-butene
Solution
trans-2-butene is maximum stable. Therefore, heat
of hydrogenation will be least.
Hence, the answer is (c).
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Class Exercise - 6
Solution
3,3-dimethyl but-1-ene
Hence, the answer is (b).
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Class Exercise 7
Which of the following is a false statement? (a)
There is sp3 hydridization in propane(b) Ethyne
has a linear structure(c) There is sp2
hybridization in ethylene(d) Alkynes show
geometric isomerism
Solution
Alkynes do not show geometrical isomerism because
it is a linear structure.
Hence, the answer is (d).
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Class Exercise - 8
Which of the following types of isomerism is not
exhibited by alkenes?(a) Position
isomerism (b) Chain isomerism(c) Geometric
isomerism (d) Metamerism
Solution
Metamerism will not be shown by alkenes.
Hence, the answer is (d).
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Class Exercise - 9
Solution
No hyperconjugation.
Hence, the answer is (c).
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Class Exercise 10
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Solution
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