Haloalkanes and Haloarenes Important Questions

Very Short Answer Questions [1 Marks Questions]

Ques. A solution of KOH hydrolyses CH₃CHClCH₂CH₃ and CH₃CH₂CH₂CH₂Cl. Which one of these is more easily hydrolyzed? (CBSE Delhi 2010)

Ans. In an aqueous solution of KOH, SN₁ mechanism takes place and as a result, the formed carbocation is stabilized. Thus 2° carbocation is a more stable carbocation than 1°.

Ques. Give the IUPAC name of the following compound: (CBSE All India 2010)
Ans. 

Ques. Write the IUPAC name of the following compound: (CH₃)₃ CCH₂Br (CBSE Delhi 2010)

Ans.
IUPAC name: 1-bromo-2, 2-dimethyl propane

Ques. Write the IUPAC name of the following compound: CH₂ = CHCH₂Br (CBSE All India 2010)

Ans.
IUPAC name: l-Bromo-prop-2-ene

Ques. Write the IUPAC name of the following compound:  (CBSE All India 2010)
Ans. IUPAC name: 1 -Bromo-2~methylprop-2-ene.

Ques. Give the IUPAC name of the following compound. (CBSE Delhi 2012)

Ans.

IUPAC name: 3-bromo-2-methyl propene

Ques. What happens when bromine attacks (CBSE All India 2012)
CH₂ = CH – CH₂ – C ≡ CH

Ans. 

Ques. Write the IUPAC name of the following:  (CBSE All India 2012)

Ans. IUPAC name: 3 Bromo-2-methyl propene.

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Short Answer Questions [2 Marks Questions]

Ques. Haloalkanes undergo nucleophilic substitution whereas haloarenes undergo electrophilic substitution. Explain. (CBSE Compartment Delhi 2012)

Ans. Haloarenes (for example, chlorobenzene) is a resonance hybrid of the following five resonant structures:

Resonance leads to a lowering of energy and hence greater stability. On the other hand, no such resonance is possible in haloalkanes. Halogens that are directly attached to the benzene ring are o, p-directing in electrophilic substitution reactions. This is due to greater electron density at these positions in resonance.

Ques. Chlorobenzene is extremely less reactive toward a nucleophilic substitution reaction. Give two reasons for the same. (CBSE Delhi 2013)

Ans. The reasons for chlorobenzene being extremely less reactive towards a nucleophilic substitution reaction are:

1. Due to resonance/diagrammatic representation, the C – Cl bond acquires a partial double bond character. As a result, the C – Cl bond in chlorobenzene is shorter and hence stronger. Thus, cleavage of the C – Cl bond in benzene becomes difficult which makes it less reactive toward nucleophilic substitution reaction.
2. Due to repulsion between the nucleophile and electron-rich arenes.

Ques. (a) Why does p-dichlorobenzene have a higher m.p. than its o- and m-isomers?
(b) Why is (±)-Butan-2-ol optically inactive? (CBSE Delhi 2013)

Ans.

1. p-isomers are comparatively more symmetrical and fit closely in the crystal lattice, thus requiring more heat to break these strong forces of attraction. Therefore, higher melting point than o- and m-isomers.
2. (±)-Butan-2-ol is optically inactive because in the racemic mix one type of rotation is cancelled by the other.

Ques. Account for the following:

1. The C – Cl bond length in chlorobenzene is shorter than that in CH₃ – Cl.
2. Chloroform is stored in closed dark brown bottles. (CBSE Delhi 2013)

Ans.

1. In haloalkanes, the halogen atom is attached 
   
   to sp3-hybridized carbon while in haloarenes it is attached to sp₂ -hybridized carbon whose size is smaller than sp3 orbital carbon. Therefore C – Cl bond in chloro-benzene is shorter than in alkyl chloride.
2. CHCl₃ is stored in dark coloured bottles to cut off light because CHCl₃ is slowly oxidised by air in presence of light to form an extremely poisonous gas, carbonyl chloride, popularly known as phosgene.

Ques. Give chemical tests to distinguish between the following pairs of compounds:

1. Benzyl chloride and Chlorobenzene
2. Chloroform and Carbon tetrachloride (CBSE Compartment Delhi 2013)

Ans.

1. Chlorobenzene and Benzyl chloride :
   Benzyl chloride is more reactive than chlorobenzene towards nucleophilic substitution reactions, therefore, benzyl chloride on boiling with aqueous KOH produces benzyl alcohol and KCl.
   
   The reaction mixture on acidification with dil. HNO₃ followed by treatment with AgNO₃ solution produces white ppt. of AgCl due to the formation of KCl

But chlorobenzene does not undergo hydrolysis under these mild conditions to give phenol and KCl.

2. Chloroform and Carbon tetrachloride

By Carbylamine test: CCl₄ does not give this reaction but chloroform gives this reaction and produces an offensive smell of phenyl isocyanide.

Ques. Explain why:

1. The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride?
2. Alkyl halides, though polar, are immiscible with water? (CBSE Compartment All India 2013)

Ans.

1. Chlorobenzene has a lower dipole moment than cyclohexyl chloride due to the lower magnitude of negative charge on the Cl atom and shorter C – Cl distance. Due to greater S-character, an sp₂-hybrid carbon is more electronegative than an sp₃-hybrid carbon. Therefore, the sp₂-hybrid carbon of C – Cl bond in chlorobenzene has less tendency to release electrons to Cl than an sp3 hybrid carbon of cyclohexyl chloride.

2. Alkyl halides and polar molecules are held together by dipole-dipole interaction. The molecules of H2O are held together by H- bonds. Since the new forces of attraction between water and alkyl halide molecules are weaker than the forces of attraction already existing between alkyl halide-alkyl halide molecules and water-water molecules, therefore alkyl halides are immiscible (not soluble) with water.

Ques. What are ambident nucleophiles? Explain with an example. (CBSE Compartment. All India 2014)

Ans. An ambident nucleophile is a nucleophile that can form new bonds at two or more spots in its structure, usually due to resonance contributors. Example: S = C = N– can act as a nucleophile with either the S or N attacking.

Also Read:

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Long Answer Questions [3 Marks Questions]

Ques.

1. State one use each of DDT and iodoform.
2. Which compound in the following couples will react faster in SN2 displacement and why?

(a) 1-bromopentane or 2-bromopentane

(b) l-bromo-2-methylbutane or 2-bromo-2-methylbutane. (CBSE Delhi 2010)

Ans:

1. Use of DDT (Dichlorodiphenyl Trichloroethane): As a powerful insecticide, it is widely used for sugarcane and fodder crops to kill mosquitoes and lice which carry pathogens.
   Use of iodoform (CHI₃): It is used as an antiseptic for dressing wounds. Its antiseptic action is due to the liberation of iodine when iodoform comes in contact with skin but not due to iodoform itself.

2. In SN₂ reactions, reactivity depends upon steric hindrance

(a) 1-Bromopentane (1° halogen) has less steric hindrance, therefore, is more reactive than 2-Bromopentane hence undergoes SN₂ reactions faster.

(b) 1-Bromo-2-methylbutane having less steric hindrance, is thus more reactive towards SN₂ reaction than 2-bromo-2-methyl butane (more steric hindrance).

Ques. Answer the following:

1. Haloalkanes easily dissolve in organic solvents, why?
2. What is known as a racemic mixture? Give an example.
3. Of the two bromoderivatives, C₆H₅CH (CH₃)Br and C₆H₅CH(C₆H₅)Br, which one is more reactive in SN₁ substitution reaction and why? (CBSE Delhi 2011)

Ans.

1. Because the new forces of attraction set up between haloalkanes and solvent molecules are of the same strength as the forces of attraction being broken.
2. A mixture that contains the equal proportions of two enantiomers of a compound in equal proportions is called a racemic mixture

Example: (±) butan-2-ol

3. Since the reactivity of SN1 reactions increases as the stability of intermediate carbocation increases. Of the two 2° bromides, the carbocation intermediate derived from C₆H-CH(C₆H₅)Br i.e. C₆H₅CHC₆H₅ is more stable as compared to the carbocation C₆H+5CHCH₃ obtained from C₆H₅CH(CH₃)Br because it is stabilized by two phenyl groups due to resonance.

Ques. Account for the following:

1. The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
2. Alkyl halides, though polar, are immiscible with water.
3. Grignard’s reagents should be prepared under anhydrous conditions. (CBSE Compartment All India 2012)

Ans.

1. Electron pairs of Cl atom are in conjugation with n electrons of the benzene ring so the C-Cl bond in chlorobenzene
   acquires some double bond character while the C-Cl bond in cyclohexyl chloride is a pure single bond.
   C – Cl bond in chlorobenzene is shorter than in cyclohexyl chloride. Since the dipole moment is a product of charge and distance, chlorobenzene has a lower dipole moment than cyclohexyl chloride.
2. Alkyl halides are polar molecules; therefore, their molecules are held together by dipole-dipole attraction. The molecules of H2O are held together by H-bonds. Since the new forces of attraction between water and alkyl halide molecules are weaker than the forces of attraction already existing between alkyl halide- alkyl halide molecules and water- water molecules, therefore, alkyl halides are immiscible with water.
3. Grignard’s reagents are very reactive. They react with alcohol, water, amines etc. to form the corresponding hydrocarbon.

R-MgX + HOH → RH + Mg(OH)X
Therefore, Grignard’s reagents must be prepared under anhydrous conditions.

Ques. Give reasons:

1. n-Butyl bromide has a higher boiling point than f-butyl bromide.
2. Racemic mixture is optically inactive.
3. The presence of nitro group (-NO₂) at o/p positions increases the reactivity of haloarenes towards nucleophilic substitution reactions. (CBSE Delhi 2015)

Ans.

1. n-Butyl bromide has a higher boiling point than f-butyl bromide because it has a larger surface area and hence has more Van der Waals forces.
2. Rotation due to one enantiomer is cancelled by another enantiomer.
3. The presence of nitro group (-NO₂) at ortho and para positions withdraws the electron density from the benzene ring and thus facilitates the attack of nucleophiles.

Ques. Give reasons: (CBSE Delhi 2016)

1. C—Cl bond length in chlorobenzene is shorter than C—Cl bond length in CH₃—Cl.
2. The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
3. SN₁ reactions are accompanied by racemization in optically active alkyl halides.

Ans.

1. In haloalkanes, the halogen atom is attached 
   to sp₃-hybridized carbon while in haloarenes it is attached to sp2 -hybridized carbon whose size is smaller than sp₃ orbital carbon. Therefore C – Cl bond in chloro-benzene is shorter than alkyl chloride.
2. Dipole moment is the product of charge and internuclear distance. In case of cyclohexyl chloride carbon—chlorine band length is longer than C—Cl band length in chlorobenzene. Hence dipole moments of cyclohexyl chloride are more.
3. SN₁ reactions are accompanied by racemization in optically active alkyl halides because the nucleophile will have an equal opportunity to attack on sp₂ hybridised carbocation from either side to give a racemic mixture.

Example:

Ques. (a) Account for the following:

(i) Electrophilic substitution reactions in haloarenes occur slowly.

(ii) Haloalkanes, though polar, are insoluble in water.

(b) Arrange the following compounds in increasing order of reactivity towards SN2 displacement:

2-Bromo-2-Methylbutane, 1-Bromopentane, 2-Bromopentane (CBSE Compartment All India 2017)

Ans. (a) (i) Due to -I effect of the halogen atom, it withdraws electrons from the benzene ring and thus the ring gets deactivated.

(ii) They fail to form hydrogen bonds with water. More energy is required to break hydrogen bonds in water and less energy is released when new attractions are set up.

(b) 2-Bromo-2-Methylbutane < 2-Bromopentane < 1-Bromopentane

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Very Long Answer Questions [5 Marks Questions]

Ques. Some alkyl halides undergo substitution whereas some undergo elimination reactions on treatment with bases. Discuss the structural features of alkyl halides with the help of examples that are responsible for this difference.

Ans. The structure of the alkyl halide as well as the chemicals used in the reaction is responsible for the reaction that will be taking place, be it substitution or elimination. The SN2 mechanism, which involves cleavage of the halide from the carbon atom and simultaneous attachment of the attacking nucleophile, is preferred by primary alkyl halides for substitution reactions.

Tertiary halides, on the other hand, go through an elimination reaction due to the creation of stable carbocation. Tertiary halides also prefer to go through the SN1 reaction, which is a two-step reaction including the production of a stable carbocation once the halide atom is cleaved.

The following reaction is for a primary alkyl halide to undergo SN₂ substitution.

The described reaction is for an SN₁ substitution of a tertiary alkyl halide. The strength of the base employed determines the mechanism. When using a weak base, such as aqueous KOH, the reaction is a substitution, whereas if we use a strong base, such as an alcoholic base, the reaction is elimination.

Ques. Some halogen containing compounds are useful in daily life. Some compounds of this class are responsible for the exposure of flora and fauna to more and more UV light which causes destruction to a great extent. Name the class of these halo compounds. According to you, what should be done to minimise the harmful effects of these compounds?

Ans. The following are some halogen-containing chemicals that are useful in daily life. Dichloromethane is used as a paint stripper, a propellant in aerosols, and a process solvent in the pharmaceutical industry. It’s also utilised as a cleaning and finishing solution for metals. Trichloromethane is used to dissolve lipids, alkaloids, iodine, and other compounds. Triiodomethane is a type of antiseptic. It has now been substituted by alternative chemicals because of its unpleasant odour.

However, several compounds in this class are responsible for posing harm to flora and wildlife by increasing amounts of UV radiation, which causes significant damage.

(i) Tetrachloromethane

When CCl₃ is discharged into the atmosphere, it begins to deplete the ozone layer. As a result, UV radiation will easily reach the earth, causing harm to humans. This will result in skin cancer, functioning disorders, eye illnesses, and immune system harm. These UV rays not only harm humans but also plants and other animals.

(ii) Freons

Freon-113 when released into the atmosphere and will rise to the top. It produces Cl atoms here in order to deplete the ozone layer. As a result of this ozone depletion, UV rays reach our atmosphere and cause significant damage.

(iii) p – p’ – Dichlorodiphenyltrichloroethane (DDT)

For the reason that DDT is not totally biodegradable, it cannot be eliminated and removed from the atmosphere. It forms a chain when dissolved in lipids. When DDT enters a human’s body, it causes an effect on the reproductive system.

We should reduce the usage of the following compounds, namely freons, hydrofluorocarbons, and fluorocarbons, in order to eliminate their detrimental impacts.

Ques. Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

Ans. Because of the following reasons, aryl halides are less reactive in nucleophilic substitution reactions.

• The lone pair of electrons on the halogen is in resonance with the benzene ring in haloarenes. As a result, the C-Cl bond gains a partial double bond character, which strengthens the bond. As a result, they have lower reactivity to nucleophilic substitution reactions.
• The carbon atom linked to the halogen is sp₂ hybridised in haloarenes. Compared to sp₃ hybridised carbon, sp₂ hybridised carbon is more electronegative. In haloarenes, the sp2-hybridised carbon can retain the electron pair of the C-X bond more tightly, making the C-CI link shorter than the C-Cl bond in haloalkanes.
• Because a shorter bond is more difficult to break than a longer connection, haloarenes are less reactive than haloarenes.
• The phenyl cation will not be stabilised by resonance in haloarenes, hence the SN₁ mechanism is ruled out (iv) Because the nucleophile and electron-rich arenes repel each other, aryl halides are less reactive than alkyl halides.

When an electron withdrawing group (NO₂) is present at the ortho and para positions, the reactivity of aryl halides can be increased. The presence of this electron withdrawing group at the above-mentioned location causes the benzene ring to lose electron density. As a result, the nucleophile will have an easier time attacking. A carbocation is produced as a result of resonance. The electron density is clearly higher at the ortho and para locations. As a result, the presence of EWG facilitates nucleophilic at ortho and para locations but not at meta.

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