Prepare for PTB exams with solved exercises on Alkyl Halides. Focus on key topics like structure, reactions, and applications to enhance your understanding and perform well in exams.
Q.4. Define alkyl halide. Which is the best method of preparing alkyl halides?
Alkyl Halide:
Alkyl halides (also known as haloalkanes) are organic compounds in which one or more halogen atoms (fluorine, chlorine, bromine, or iodine) are covalently bonded to an alkyl group (an aliphatic carbon chain). They have the general formula ( R-X ), where ( R ) represents the alkyl group, and ( X ) represents the halogen atom.
Best Method of Preparing Alkyl Halides:
The best method to prepare alkyl halides is by the free radical halogenation of alkanes. This process involves substituting a hydrogen atom in an alkane with a halogen atom under the influence of heat or light (UV radiation).
General Reaction:
R-H + X2 → (Heat or UV) R-X + H-X
Where ( R-H ) is an alkane, and ( X2 ) is a halogen molecule such as ( Cl2 ) or ( Br2 ). This process produces alkyl halides and a hydrogen halide as a byproduct.
Q.5. Write down a method for the preparation of ethyl magnesium bromide in the laboratory.
Preparation of Ethyl Magnesium Bromide:
Ethyl magnesium bromide is a Grignard reagent, which can be prepared by reacting ethyl bromide with magnesium in the presence of dry ether.
Reaction:
C2H5-Br + Mg → (Dry ether) C2H5MgBr
Procedure:
- Place magnesium turnings in a dry flask.
- Add dry ether to the flask to keep the reaction environment anhydrous.
- Slowly add ethyl bromide (C2H5Br) to the flask while stirring the mixture.
- The reaction begins with the formation of ethyl magnesium bromide (C2H5MgBr).
- The reaction is exothermic, so it is crucial to maintain the temperature by adding the reactants slowly.
Grignard reagents are highly reactive and must be prepared in an anhydrous environment because they react with water.
Q.6. Give IUPAC names to the following compounds.
i.
Structure: CH3-CH2-CH2-CH2-CH2-Cl
IUPAC Name: 1-Chloropentane
ii.
Structure: CH3CH2CH2CH3
IUPAC Name: Butane
iii.
Structure: CH3-CH(CH3)-CH2-CH2-CH2-CH3
IUPAC Name: 2-Methylhexane
iv.
Structure: CH2(Cl)-CH(CH3)-CH3
IUPAC Name: 1-Chloro-2-methylpropane
v.
Structure: ( \text{CH}_3\text{CBr}_3 )
IUPAC Name: Bromoform
vi.
Structure: ( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl} )
IUPAC Name: 1-Chlorobutane
vii.
Structure: ( \text{CH}_3\text{CH}_2\text{CHClCH}_2\text{CH}_2\text{CH}_2\text{Cl} )
IUPAC Name: 1,6-Dichlorohexane
viii.
Structure:
( \text{Br-CH}_2\text{CH}_3 )
IUPAC Name: Ethyl bromide or Bromoethane
ix.
Structure: CH2(Cl)-CH2-CH3
IUPAC Name: 1-Chloropropane
x.
Structure: CH2(Br)-C(CH3)2-CH3
IUPAC Name: 1-Bromo-2,2-dimethylpropane
xi.
Structure: CH2(Cl)-CH(CH3)-CH2-CH3
IUPAC Name: 1-Chloro-2-methylbutane
xii.
Structure: CH3-C(CH3)2-CH2-CH3
IUPAC Name: 2,2-Dimethylbutane
xiii.
Structure: CH3-C(Br)(CH3)-CH3
IUPAC Name: Bromotert-butane or 2-Bromo-2-methylpropane
Q.7. Draw all the possible structures that have the molecular formula ( C4H9Cl ). Classify each as primary, secondary, or tertiary chloride. Give their names according to the IUPAC system.
- 1-Chlorobutane (Primary chloride):
CH2(Cl)-CH2-CH2-CH3 - 2-Chlorobutane (Secondary chloride):
CH3-CH(Cl)-CH2-CH3 - 2-Chloro-2-methylpropane (Tertiary chloride):
CH3-C(Cl)(CH3)-CH3 - 1-Chloro-2-methylpropane (Primary chloride):
CH2(Cl)-CH(CH3)-CH3
Q.8. Using ethyl bromide as a starting material, how would you prepare the following compounds?
(a) n-Butane:
Ethyl bromide + sodium metal → n-Butane (Wurtz reaction).
Reagent: Na, Condition: Dry ether
(b) Ethyl alcohol:
Ethyl bromide + aqueous KOH → Ethyl alcohol
Reagent: Aqueous KOH, Condition: Reflux
(c) Ethyl cyanide:
Ethyl bromide + KCN → Ethyl cyanide
Reagent: KCN, Condition: Alcoholic
(d) Ethane:
Ethyl bromide + Zn/HCl → Ethane (Reduction reaction)
Reagent: Zn/HCl
(e) Ethene:
Ethyl bromide + alcoholic KOH → Ethene (Dehydrohalogenation)
Reagent: Alcoholic KOH, Condition: Heat
(f) Propanoic acid:
Ethyl bromide + KCN → Ethyl cyanide → Hydrolysis → Propanoic acid
Reagent: KCN and H3O+
(g) Propane:
Ethyl bromide + Mg → Grignard reagent → Addition of water → Propane
Reagent: Mg, Condition: Dry ether, followed by H2O
Q.9. Write a detailed note on the mechanism of nucleophilic substitution reactions.
Nucleophilic substitution reactions involve the replacement of a leaving group (usually a halide ion) by a nucleophile. There are two main types of nucleophilic substitution mechanisms:
- SN1 Mechanism (Unimolecular Nucleophilic Substitution):
- Occurs in two steps.
- Step 1: Formation of a carbocation after the leaving group departs.
- Step 2: Nucleophile attacks the carbocation to form the product.
- Occurs in tertiary alkyl halides due to the stability of the carbocation.
- SN2 Mechanism (Bimolecular Nucleophilic Substitution):
- Occurs in one concerted step.
- The nucleophile attacks from the opposite side of the leaving group, leading to inversion of configuration.
- Occurs primarily in primary and secondary alkyl halides.
Q.10. What do you understand by the term β-elimination reaction? Explain briefly the two possible mechanisms of 3-elimination reactions.
β-Elimination refers to a reaction where a hydrogen atom from the β-carbon (the carbon adjacent to the carbon bonded to the leaving group) and the leaving group are removed, resulting in the formation of a double bond.
Two possible mechanisms for β-elimination are:
- E1 Mechanism (Unimolecular Elimination):
- Occurs in two steps: first, the leaving group departs to form a carbocation, and then the base removes a proton from the β-carbon, resulting in an alkene.
- Typically seen in tertiary halides.
- E2 Mechanism (Bimolecular Elimination):
- Occurs in a single step: the base simultaneously abstracts a proton from the β-carbon as the leaving group leaves, resulting in the formation of an alkene.
- Occurs in primary and secondary halides.
Q.11. What products are formed when the following compounds are treated with ethyl magnesium bromide, followed by hydrolysis in the presence of an acid?
i) HCHO (Formaldehyde) → 1-Propanol
ii) CH3CHO (Acetaldehyde) → 2-Propanol
iii) CO2 → Propanoic acid
iv) (CH3)2CO (Acetone) → Tert-Butyl alcohol
v) CH3—CH2—CHO (Propanal) → 2-Butanol
vi) ClCN → Propanoic acid (after hydrolysis of nitrile)
Q.12. How will you carry out the following conversions?
i) CH4 → CH3COOH
Method: First, convert methane to methyl chloride (CH3Cl) by chlorination. Then, convert CH3Cl to acetic acid (CH3COOH) via carboxylation using a Grignard reagent and carbon dioxide.
ii) CH3—CH3 → (CH3—CH2)2N+ Br
Method: Ethane can be halogenated to form ethyl bromide (CH3CH2Br), which can then be reacted with ammonia to form a quaternary ammonium salt.
iii) CH2 = CH2 → CH3—CH2—CH2—CH2—OH
Method: Ethene can be reacted with HBr to form ethyl bromide, which can then be converted to 1-butanol through Wurtz coupling and hydration.
iv) CH3—CH2Cl → CH3—CH=CH2
Method: Ethyl chloride can undergo dehydrohalogenation with alcoholic KOH to form propene.
v) CH3COOH → CH3—CH=CH2
Method: Decarboxylation of acetic acid leads to the formation of propene.
