About 45% of the questions are asked from Organic Chemistry in any engineering entrance examination. Students should understand the mechanics of all the reactions in organic Chemistry. If students try to do rote learning instead of understanding the mechanism, then they can’t answer the questions which are asked in the examination.
The topic ‘Alkyl Halides’ plays an important role in securing good marks in Organic Chemistry. About 1-2 questions are being asked from this topic in the examination.
In this article, we bring to you the notes of chapter Alkyl Halides including important concepts, chemical properties, physical properties and some solved questions for UPSEE/UPTU 2018.
About the notes:
1. These notes are prepared by Subject Experts of Chemistry.
2. Some solved questions are included in these notes to make engineering aspirants understand the concepts in an easier manner.
3. It will help aspirants to save their precious time while preparing for UPSEE entrance examination 2018.
Students can read the following topics here:
1. Classification of haloalkanes and haloarenes,
2. Preparation of alkyl halides and
(a) From Alcohols and
(b) From Hydrocarbons
(i) By free radical halogenation,
(ii) By electrophilic substitution,
(iii) Sandmeyer's reaction,
(iv) Addition of halogens,
(v) Hunsdiecker reaction,
(vi) Allylic Bromination and
(vii) Halogen exchange
3. Chemical properties of Alkyl Halides
(a) Nucleophilic substitution
(i) Substitution nucleophilic bimolecular (SN2) and
(ii) Substitution nucleophilic unimolecular (SN1)
(b) Elimination reactions
(i) E1 and
(ii) E2
UPSEE/UPTU Chemistry Syllabus: 2017 - 2018
Important Concepts:
Chemical Reactions of Alkyl Halides
The reaction can be broadly classified in two categories:
(a) Nucleophilic substitution
(b) Elimination reactions
Nucleophilic substitution reactions: In this reaction a nucleophile, which is rich in electrons, attacks partial positive charge on the carbon atom bonded to halogen to replace the leaving group.
Nucleophilic reactions proceed by two different mechanism:
(a) Substitution nucleophilic bimolecular (SN2)
(b) Substitution nucleophilic unirnolecular (SN1)
- The reaction follows second order kinetics
- No intermediate is formed.
- It usually requires a strong nucleophile.
- The order of reactivity followed as:
Primary halide > Secondary halide > Tertiary halide
- It is carried out in polar protic solvents (water, Alcohol, acetic acid etc.).
- These reactions occur in two steps as shown above
- The order of leaving ability is:
F- < Cl- < Br- < F-
- The order of reactivity is as shown below:
Difference between E1 and E2 reaction mechanism:
| Attributes |
E1 |
E2 |
| Rate law
|
Depend on the concentration of substrate |
Depends on the concentration of both substrate and base |
| Barrier |
Formation of carbocation 3o>2o>> 1o |
None |
| Base |
Does not require strong base |
Requires strong base |
| Stereochemistry |
Does not require stereochemistry |
Leaving group must be anti to hydrogen removed |
UPSEE: Important Questions and Preparation Tips – Solutions
Some solved questions are given below:
Question 1:
Which one is least reactive in a nucleophilic substitution reaction?
(a) CH3CH2CI
(b) CH2=CHCH2CI
(c) CH2=CHCI
(d) (CH3)3CCI
Solution 1:
Vinyl chloride is least reactive for nucleophilic substitution reaction due to resonance
stabilisation of C-Cl bond.
Hence, the correct option is (c).
You can also have a look on the following questions and solutions to check your preparation.
Question 1:
Which is the correct increasing order of boiling points of the following compounds?
1-bromoethane, 1-bromopropane, 1-bromobutane, Bromobenzene
(a) Bromobenzene < 1-bromobutane < 1-bromopropane < 1-bromoethane
(b) Bromobenzene < 1-bromothane < 1-bromopropane < 1-bromobutane
(c) 1-bromopropane < 1-bromobutane < 1-bromoethane < Bromobenzene
(d) 1-bromoethane < 1-bromopropane < 1-bromobutane < Bromobenzene
Solution 1:
Boiling point increases with increase in size of hydrocarbon part for the same haloalkanes. All the given haloalkenes contain same halogen atom i.e., bromine but the number of carbon atoms in hydrocarbon part of the molecule are increasing from ethane to benzene.
So the boiling point is minimum for 1-bromoethane and maximum for 1-bromobenzene.
Hence, the correct option is (d).
UPSEE: Important Questions and Preparation Tips – States of Matter
Question 2:
Alkyl fluorides are synthesised by alkyl chloride/bromide in presence of ……or……….
(a) CaF2, Hg2F2
(b) CoF2, NaF
(c) Hg2F2, CoF2
(d) NaF, Hg2F2
Solution 2:
Alkyl fluorides are synthesised by alkyl chloride/bromide in presence of CoF2 or Hg2F2. Only transition metal fluorides react with alkyl chloride/bromide to form alkyl fluorides. Alkali metal fluoride such as NaF and alkaline earth metal fluoride such as CaF2 do not react to form fluorides.
Hence, the correct option is (a).
Question 3:
Ethylidene chloride is a/an…….
(a) vic-dihalide
(b) gem-dihalide
(c) allylic halide
(d) vinylic halide
Solution 3:
In vic-dihalides, halongen atom are present on the adjacent carbon atoms.
In gem-dihalides, halogen atoms are present on the same carbon atom. They are known as alkylidene halides
In allylic halides, halogen atom is bonded to sp3 hybridised carbon atom next to carbon-carbon double bond.
in vinylic halides, halogen atom is bonded to sp2 hybridised carbon atom of a carbon-carbon double bond.
In ethylidene chloride (H3C-CHCI2) both halogen atoms are present on same carbon atom so it is gem-dihalide.
Hence, the correct option is (b).
Question 4:
The synthesis of alkyl fluorides is best accomplished by
(a) Free radical fluorination
(b) Sandmeyer's reaction
(c) Finkelstein reaction
(d) Swarts reaction
Solution 4:
Option (a)
Free radical fluorination is highly explosive reaction therefore it is not preferred for the preferred for the synthesis of alkyl fluorides.
Option (b)
Sandmeyer’s reaction is used to synthesize aryl halides from aryl diazonium salts.
Option (c)
Finkelstein reaction is used mainly for the preparation of alkyl iodides.
Option (d)
The synthesis of alkyl fluoride is best accompanied by Swarts Reaction. Alkyl fluoride is synthesized by heating alkyl bromide and alkyl chloride in the presence of AgF, SbF3, Hg2F2 etc.
Hence, the correct option is (d).
Conclusion:
Students can easily understand about the preparation, chemical properties and physical properties of Alkyl Halides with the help of these notes. Also, it will help students in understanding the mechanism of name reactions related to Alkyl Halides.
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