2nd PUC Chemistry Question Bank Chapter 10 Haloalkanes and Haloarenes

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Karnataka 2nd PUC Chemistry Question Bank Chapter 10 Haloalkanes and Haloarenes

Question 1.
What are haloalkanes? Give examples.
Answer:
Haloalkanes (alkyl halide) are halogen derivatives of alkanes with general formula. C_nH_2n+1 X

Question 2.
What are haloarenes? Give examples.
Answer:
Haloarenes (aryl halides) are halogen derivatives of arenes (aromatic compounds).
Ex:

Question 3.
In aryl halides, what is the hybridisation of carbon atom to which halogen is attached?
Answer:
sp²

Question 4.
Give IUPAC name of CH₂ = CHCl.
Answer:
Chloroethene.

Question 5.
How alkyl halides are obtained from alcohols?
Answer:
Alkyl halides are obtained from alcohols.

Question 6.
Write the general equation for the reaction of primary alochol with SOCl₂.
Answer:
R – OH + SOCl₂ → R – Cl + SO₂ + HCl

Question 7.
Explain Finkelstein reaction.
Answer:
Alkyl iodides are prepared by the reaction of alkyl chlorides or alkyl bromides with Nal in dry acetone.

Question 8.

Answer:
Finkelstein reaction.

Question 9.
Explain Swartz reaction.
Answer:
Alkyl fluorides are prepared by heating alkyl chloride or alkyl bromide with metallic fluorides like AgF.
CH₃Br + AgF → CH₃F + AgBr

Question 10.
Identify the product A,B and C in the following reaction.

Answer:
A = CH₃Cl
B = CH₃I
C = C₆H₅CH₃

Question 11.
CH₃Br + AgF → CH₃F + AgBr. Name the reaction.
Answer:
Swartz reaction.

Question 12.
Boiling points of alkyl halides are greater than their parent hydrocarbon. Give reason.
Answer:

• Due to polar nature of alkylhalides dipole-dipole interaction increases.
• Due to increase in molecular mass van der Waals forces increases. Therefore boiling point increases.

Question 13.
Which of the following isomeric haloalkanes have low boiling point? Give reason.

Answer:
Isomeric haloalkane ‘C’ has low boiling point. Increase in branching decreases the boiling point.

Question 14.
Why is boiling point of ethyl bromide is greater than that of ethyl chloride.
Answer:
Ethyl bromide has higher magnitude of van der Waals forces due to its high molecular mass.

Question 15.
para-dichiorobenzene has higher melting point than those of ortho and meta Isomers. Give reason.
Answer:
para-dichlorobenzene has symmetry in the molecule. This leads to close packing in the crystal. Hence it has high melting point than ortho and para isomer.

Question 16.

Answer:
CH₃CH₂Br + AgCN → CH₃CH₂NC + AgBr

Question 17.
Give examples to show that alkyl halides undergoes nucleophilic substitution reactions.
Answer:

Question 18.
Explain S_N – 1 reaction mechanism.
Answer:
The reaction between tertbutyl bromide and hydroxide ion gives tert-butyl alcohol. The rate of reaction depench only on the concentration of tert-butyl bromide. Hence it is a nucleophillic first order substitution reaction.

Mechanism:
Step 1: Polarised C – Br undergoes cleavage to produce planar carbocation,

Step 2: The carbocation is then attacked by nucleophile (OH^–) on either side to form tert-butyl alcohol.

Question 19.
Explain S_N – 2 reaction mechanism?
Answer:
The reaction between methy alcohol and OH^– gives methanol and Cl^–. The rate of reaction depends on the concentration of both the reactants. Hence it is a nucleophilic second order substitution reaction.

OH^– group which approaches the methyl group from the back side starts forming covalent bond with carbon atom.

A transition state is reached in which carbon atom is simultaneously bonded to incoming nucleophile (OH^–) and out going leaving group (Cl).

As this transition state is unstable finally chlorine atom leaves as Cl^– ion and methyl alcohol is formed.

Question 20.
Give two differences between S_N – 1 and S_N – 2 mechanism.
Answer:

SN – 1	SN – 2
(a) It is a first order reaction.	(a) It is a second order reaction.
(b) It is two step mechanism.	(b) It is single step mechanism.
(c) Inversion may or may not takes place.	(c) Inversion take place.

Question 21.
What is optical activity? Explain.
Answer:
Certain compounds rotate plane polarised light either to the left or to the right, when passed through their solution. This is called optical activity. Such compounds are called optical active compounds.

If the compound rotates the plane polarised light to the right then it is called dextrorotatory (+) or the d-form. If the compound rotates the plane polarised light to the left then it is called levorotatory (-) or the 1-form.

The dextro and levo isomers of a compound are called optical isomers and phenomenon is called optical isomerism.

Question 22.
What is chiral carbon or asymmetric carbon?
Answer:
If four group around the central carbon atom are different, than (hat carbon is called asymmetric carbon or chiral carbon. The resulting molecule would lack symmetry and is called asymmetric molecule,

Question 23.
What is chirality?
Answer:
The molecules which are not super imposable on their mirror images are called chiral molecules and the property is called chirality.

Question 24.
What are enantiomers? Give example.
Answer:
The stereoisomers related to each other as non-superimposable mirror images are called enantiomers.

Question 25.
What are enantiomers?
Answer:
The stereoisomers related to each other as non-superimposable mirror images are called enantiomers.

Question 26.
What is racemic mixture?
Answer:
A mixture containing two enantiomers in equal proportions will have zero optical rotation. This mixture is called racemic mixture.

Question 27.
A racemic mixture is optically inactive. Why?
Answer:
Rotation by an enantiomer cancelled by the other.

Question 28.
Expiain elimination reaction. Give example.
Answer:
When a haloalkane is heated with alcoholic potassium hydroxide, there is elimination of hydrogen from β-carbon and a halogen atom from the ∝-carbon atom. As a result, an alkene is formed. This reaction is called β-elimination reaction or dehydrohalogenation reaction.

Question 29.
State Saytzeff rule. Give example.
Answer:
In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.
Ex: 2-Bromopentane gives pent-2-ene as the product, instead of pent-l-ene.

Question 30.
Name the reagent used in the dehydrohalogenation of haloalkanes.
Answer:
Alcoholic potash.

Question 31.
Name the major product obtained when tertiary butyl bromide is heated with alcoholic KOH.
Answer:
2-methyl propene.

Question 32.
Which gas is liberated when 2-Bromopropene is heated with alcoholic potash.
Answer:
Propylene (Propene) gas.

Question 33.
Write the IUPAC name of the major product obtained when 2-Bromopentane with alcoholic KOH. Give equation. Name the reaction.
Answer:
Pent-2-ene

Elimination reaction.

Question 34.
What are Grignard reagents? Give example.
Answer:
Organo magnesium halides are called Grignard reagents. The general formula is R – Mg – X. When alkyl halide is treated with magnesium powder in ether medium Grignard reagent is obtained.
R – X + Mg → R – Mg – X

Question 35.
Aryl halides are less reactive towards nucleophilic substitution reaction. Give four reasons.
Answer:

• Due to resonance, the electron pair of halogen atom are in conjugation with pi electrons of the ring. As a result C – X gets partial double bond character and bond breakage is difficult.
• In haloarenes, halogen bonded carbon atom is sp2 hybridised. Hence breaking of C – X bond is very difficult.
• Phenyl cation formed as a result of self-ionisation will not be stabilised by resonance.
• Nucleophilic substitution reaction is very less in electron rich arenes.

Question 36.
Give the conversion of chlorobenzene to phenol.
Answer:
When chlorobenzene is heated with aqueous sodium hydroxide solution at 623K at a pressure of 300 atm phenol is obtained.

Question 37.
Give the conversion of chlorobenzene to 2-chloroacetophenone. (Friedel Crafts acylation).
Answer:
When chlorobenzene is heated with acetyl chloride in presence of anhydrous aluminium-chloride, 2-chloroacetophenone and 4-chloroacetophenone is obtained.

Question 38.
Explain Wurtz-Fittig reaction with equation.
Answer:
A mixture of an alkyl halide and aryl halide gives an alkyl arene, when treated with sodium in dry ether.

Question 39.
Explain Fittig reaction with equation.
Answer:
When aryl halide treated with sodium in ether medium, two aryl groups join together to form biphenyl.

Question 40.
Name the organic compound formed when chlorobenzene is treated with sodium in dry ether.
Answer:
Biphenyl.

Question 41.
What is the name of the following reaction.

Answer:
Fittig’s reaction.

Question 42.
What are freons?
Answer:
Chlorofluorocarbon compounds of methane and ethane are called freons.