Brief questions and answers: Chemistry: Haloalkanes and Haloarenes

Haloalkanes and Haloarenes | Chemistry

Answer the following questions

26. Classify the following compounds in the form of alkyl, allylic, vinyl, benzylic halides

i. CH3 â€“ CH = CH – Cl

ii. C6H5CH2I

iii. 

iv. CH2 = CH – Cl

Ans:

a) CH₃-CH = CH-Cl - Vinylic halide

b) C₆H₅CH₂I - Benzylic halide

c)  - Alkyl halide

d) CH₂ = CH-Cl - Vinylic halide

27. Why chlorination of methane is not possible in dark?

Chlorination of methane is a free radical substitution reaction. If chlorination of methane is carriedout in dark, the energy will not be sufficient to undergo homolytic fission of chlorine to produce chlorine free radical.

28. How will you prepare n propyl iodide from n-propyl bromide?

When n-propyl bromide on heating with concentrate solution of sodium iodide in dry acetone forms n-propyl iodide. This reaction is called Finkelstein reaction.

CH₃CH₂CH₂Br + NaI ^__acetone__Δ^_→ CH₃CH₂CH₂I + NaBr

29. Which alkyl halide from the following pair is i) chiral ii) undergoes faster SN2 reaction?

30. How does chlorobenzene react with sodium in the presence of ether? What is the name of the reaction?

Chlorobenzene react with sodium in the presence of dry ether gives biphenyl. This reaction is called fittig reaction.

C₆H₅Cl + 2Na + Cl - C₆H₅ ^Ether_Δ^__→ C₆H₅ - C₆H₅ +2NaCl

31. Give reasons for polarity of C-X bond in halo alkane.

In haloalkane, halogen atom is more electronegative than carbon. So, the shared pair of electron lies closer to the halogen atom. As a result, the halogen carries a small negatives charge δ^− while the carbon carries a small positive charge (δ⁺). Thus, C - X bond is a polar bond.

32. Why is it necessary to avoid even traces of moisture during the use of Grignard reagent?

Grignard reagent is highly reactive towards moisture to give alkane

33. What happens when acetyl chloride is treated with excess of CH3MgI?

Acetyl Chloride reacts with excess methyl magnesium iodide to give tert - butyl alcohol.

34. Arrange the following alkyl halide in increasing order of bond enthalpy of RX

CH3Br, CH3F, CH3Cl, CH3I

The increasing order of bond enthalpy of alkyl halides are. CH₃I < CH₃Br < CH₃Cl < CH₃F

35. What happens when chloroform reacts with oxygen in the presence of sunlight?

Chloroform undergoes oxidation in the presence of light and air to form a poisonous gas, carbonyl chloride also known as phosgene.

36. Write down the possible isomers of C5H11Br and give their IUPAC and common names.

The possible isomers of C₅H₁₁Br are

37. Mention any three methods of preparation of haloalkanes from alcohols.

From alcohols:

a) Reaction with hydrogen halide: Alcohols react with hydrogen halide to yield haloalkanes.

R - OH + HX → R-X + H₂O

Example : CH₃CH₂OH + HCl ^{___anhy ZnCl2}→ CH₃CH₂C1 + H₂O

b) Reaction with Phosphurous halides :

Alcohol react with PX₅ or PX₃ to form haloalkanes.

R - OH + PX₅ → R - X + POX₃+ HX

Example : CH₃CH₂OH + PCl₅ → CH₃CH₂Cl+ POCl₃ + HCl

c) Reaction with thionyl chloride :

Alcohols are refuxed with SOCl₂ in the presence of small amount of pyridine to give chloroalkane. (Darzen's Reaction)

R - OH + SOCl₂ ^__pyridine__→ R - Cl + SO₂ + HCl

Example : CH₃CH₂OH + SOCl₂ ^__pyridine__→ CH₃CH₃Cl + SO₂ + HCl 

38. Compare SN1 and SN2 reaction mechanisms.

39. Reagents and the conditions used in the reactions are given below. Complete the table by writing down the product and the name of the reaction.

Answer:

40. Discuss  the  aromatic  nucleophilic substitutions reaction of chlorobenzene.

Aromatic nucleophilic substitution reaction :

Halo arenes do not undergo nucleophilic substitution reaction readily. This is due to C-X bond in aryl halide is short and strong and also the aromatic ring is a centre of high electron density.

The halogen of haloarenes can be substituted by OH^−, NH₃, or CN^− with appropriate nucleophilic reagents at high temperature and pressure.

Examples :

41. Account for the following

i. t-butyl chloride reacts with aqueous KOH by SN1 mechanism while n-butyl chloride reacts with SN2 mechanism.

ii. p-dichloro benzene has higher melting point than those of o-and m-dichloro benzene.

i) In general, S_N¹ reaction proceeds through the formation of carbo cation, t-butyl chloride reacts with aqueous KOH, it form tert - butyl chloride reacts with aqueous KOH, it form tert -butyl carbo cation which is more stable. Hence, it follow S_N¹ mechanism.

In the case of n-butylbromide, primary n-butyl carbo cation is formed which is least stable so, it does not follow S_N1 mechanism. Hence, it follow S_N2 mechanism.

ii) P - dichloro benzene has high melting point than the other two isomers due its symmetry which leads to more close packing of its molecules in the crystal lattice. Hence, it has greater intermolecular force of attraction. Therefore, it requires more energy for melting.

42. In an experiment ethyliodide in ether is allowed to stand over magnesium pieces. Magnesium dissolves and product is formed

a) Name the product and write the equation for the reaction.

b) Why all the reagents used in the reaction should be dry? Explain

c) How is acetone prepared from the product obtained in the experiment.

a) Methyl magnesium is formed as product.

CH₃I + Mg ^_ether_→ CH₃ MgI

b) Generally, Grignard reagent are highly reactive towards moisture to form alkane.

43. Write a chemical reaction useful to prepare the following:

i) Freon-12 from Carbon tetrachloride

ii) Carbon tetrachloride from carbon disulphide

44. What are Freons? Discuss their uses and environmental effects

The Chlorofluoro derivatives of methane and thane are called freons.

Uses : Freons are used as

i) refrigerants in refrigerators and air conditioners.

ii) Propellant for aerosols and foams.

iii) Propellant for foams to spray out deodorants, shaving creams and insecticides.

45. Predictthe products when bromoethane is treated with the following

i. KNO2

ii. AgNO2

i) Bromoethane reacts with alcoholic KNO₂ to form ethyl nitrite.

C₂H₅Br + KNO₂ → C₂H₅ - O -N = O + KBr

ii) Bromoethane react with alcoholic silver nitrite to form nitro ethane.

C₂H₅Br +AgNO₂ → C₂H₅ NO₂+AgBr

46. Explain the mechanism of SN1 reaction by highlighting the stereochemistry behind it

S_N¹ Mechanism:

S_N¹ stands for unimolecular nucleophilic substitution

'S' stands for substitution

‘N’ stands for nucleophilic

‘1’ stands for unimolecular (one molecule is involved in the rate determining step) The rate of the following S_N¹ reaction depends upon the concentration of alkyl halide (RX) and is independent of the concentration of the nucleophile (OH^−).

∴ Rate of the reaciton = k [alkyl halide].

R - Cl + OH^− → R - OH + Cl^−

This S_N1 reaction follows first order kinetics ans occurs in two steps.

Let us consider S_N1 reaction mechanism by taking a reaction between tertiary butyl bromide with aqueous KOH.

This reaction takes place in two steps as shown below

Step −1 Formation of carbocation

The polar C - Br bond breaks forming a carboncation and bromide ion. This step is slow and hence it is the rate determining step.

The carbocation has 2 equivalent lobes of the vacant 2p orbital, so it can react equally rapidly from either face.

Step - 2 Nucleophilic attack on carbocation

As nucleophilic reagent OH^− can attack carbocation from both the sides, to form equal proportion of dextro and levorotatory optically active isomers which from optically inactive racemic mixture.

47. Write short notes on the the following

i. Raschig process

ii. Dows Process

iii. Darzens process

i) Raschig process

Chloro benzene is commercially prepared by passing a mixture of benzene vapour, air and HCl over heated cupric chloride. This reaction is called Raschig process.

(ii) Dows process

Reaction of Chlorobenzene with caustic soda at 350°C and 300 atm gives phenol. This reaction is used for large scale preparation as it known as Dows process.

(iii) Darzens Process

Alcohols are refluxed with SOCl₂ in the presence of small amount of pyridine to give chloro alkanes.

R - OH + SOCl₂  ^__Pyridine_→ R - Cl + SO₂ + HCl

Example

CH₃CH₂OH Ethanol + SOCl₂ ^__Pyridine_→ CH₃CH₂Cl Chloroethane + SO₂ + HCl

This reaction is known as Darzen’s reaction.

48. Starting from CH3MgI, How will you prepare the following?

i. Acetic acid

ii. Acetone

iii. Ethyl acetate

iv. Iso propyl alcohol

v. Methyl cyanide

49. Complete the following reactions

Answer:

50. Explain the preparation of the following compounds

i) DDT

ii) Chloroform

iii) Biphenyl

iv) Chloropicrin

v) Freon-12

i) DDT

DDT can be prepared by heating a mixture of chlorobenzene with chloral (Trichlor) acetaldehyde) in the presence of con.H₂SO₄. 

ii) Chloroform

Chloroform is prepared by distilling the mixture of ethyl alcohol with a suspension of bleaching powder - The reaction proceeds in the following steps.

CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂

CH₃CH₂OH + Cl₂ → CH₃CHO + 2HCl (Oxidation)

CH₃CHO + 3Cl₂ → CCl₃CHO + 3HCI (Chlorinaiton)

2CC1₃CHO + Ca(OH)₂ → 2CHCl₃ Chloroform + (H COO)₂Ca Calcium formate

iii) Biphenyl:

Chloroform react with sodium metal in dry ether gives biphenyl.

iv) Chloropicrin:

Chloroform reacts with nitric acid to from chloropicrin.

v) Freon−12

Freon−12 is prepared by the action of hydrogen fluoride on carbon tetrachloride in the presence of antimony penta chloride (SbCl₃)

This reaction is called SWARTZ REACTION

CCl₄ + 2HF ^___SbCl3→ CCl₂F₂ + 2HCl

51. An organic compound (A) with molecular formula C2H5Cl reacts with KOH gives compounds (B) and with alcoholic KOH gives compound (C). Identify (A),(B), and (C)

Given:

Solution:

As per molecular formula, compound (A) is ethyl chloride. Ethyl chloride (A) reacts with aqueous KOH gives ethyl alcohol (B).

CH₃CH₂C1 (A) + aq. KOH → CH₃CH₂OH  (B) + KCl

 Ethyl chloride (A) with alcoholic KOH undergoes dehydrohalogenation to give ethylene (C).

CH₃CH₂Cl (A) + Alc. KOH → CH₂ = CH₂ (C) + KCl + H₂O

Result:

Compound : Formula, Name

A : CH₃CH₂Cl Ethyl chloride

B : CH₃CH₂OH Ethyl alcohol

C : CH₂ = CH₂ Ethylene

52. Simplest alkene (A) reacts with HCl to form compound (B).Compound (B) reacts with ammonia to form compound (C) of molecular formula C2H7N.Compound (C) undergoes carbylamine test. Identify (A), (B), and (C).

Given :

Solution :

Simplest alkene (A) means, it is ethylene. Ethylene (A) react with HCl to give form ethyl chloride (B)

CH₂ = CH₂ (A) + HCl → CH₃CH₂Cl (B)

Ethylchloride (B) reacts with ammonia to form ethylamine (C)

CH₃CH₂Cl (B) + NH₃ → CH₃CH₂NH₂ (C) + HCl

Result :

Compound : Formula & Name

A : CH₂ = CH₂ Ethylene

B : CH₃CH₂Cl Ethyl chloride

C : CH₃CH₂NH₂ Ethylamine

53. A hydrocarbon C3H6 (A) reacts with HBr to form compound (B). Compound (B) reacts with aqueous potassium hydroxide to give (C) of molecular formula C3H6O.what are (A) (B) and (C). Explain the reactions.

Solution:

As per molecular formula, A must be alkene ie., propene compound (A) reacts with HBr forms isopropyl bromide (B)

Isopropyl bromide (B) reacts with aq. KOH give isopropyl alcohol (C)

Result:

54. Two isomers (A) and (B) have the same molecular formula C2H4Cl2. Compound (A) reacts with aqueous KOH gives compound (C) of molecular formula C2H4O. Compound (B) reacts with aqueous KOH gives compound (D) of molecular formula C2H6O2. Identify (A),(B),(C) and (D).

Given:

Solution:

Compound (A) is dihalide. So, the possible dihalide will be vicinal dihalide and gemdihalide. The possible two isomers are

Compound (A) 1, 1-Dichloro ethane on hydrolysis with aq.KOH give aldehyde (B)

Compound (B), 1, 2 - Dichloro ethane on hydrolysis with aq.KOH gives diols (D) 

Result: