HydrocarbonsClass 11 Chemistry NCERT Solutions
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Q1In-text Problems
Write structures of different chain isomers of alkanes corresponding to the molecular formula . Also write their IUPAC names.
Solution
To Find: Structures and IUPAC names of chain isomers of .
Solution:
The molecular formula corresponds to hexane. There are five chain isomers of hexane.
(i)
A straight chain of six carbon atoms:
IUPAC Name: Hexane (or n-Hexane)
(ii)
A chain of five carbon atoms with one methyl group as a branch:
The methyl group can be at position 2 or 3.
- At position 2: IUPAC Name: 2-Methylpentane
- At position 3: IUPAC Name: 3-Methylpentane
(iii)
A chain of four carbon atoms with two methyl groups as branches:
The methyl groups can be at positions 2,2 or 2,3.
- At positions 2,3: IUPAC Name: 2,3-Dimethylbutane
- At positions 2,2: IUPAC Name: 2,2-Dimethylbutane
Final Answer: The five chain isomers of are Hexane, 2-Methylpentane, 3-Methylpentane, 2,3-Dimethylbutane, and 2,2-Dimethylbutane.
Q2In-text Problems
Write structures of different isomeric alkyl groups corresponding to the molecular formula . Write IUPAC names of alcohols obtained by attachment of -OH groups at different carbons of the chain.
Solution
To Find: Structures of isomeric alkyl groups of and the IUPAC names of the corresponding alcohols.
Solution:
There are 8 possible isomeric alkyl groups with the formula . The corresponding alcohols are:
- From n-pentane skeleton:
(i)
Alkyl group:
Corresponding alcohol:
Name of alcohol: Pentan-1-ol
(ii)
Alkyl group:
Corresponding alcohol:
Name of alcohol: Pentan-2-ol
(iii)
Alkyl group:
Corresponding alcohol:
Name of alcohol: Pentan-3-ol
- From isopentane (2-methylbutane) skeleton:
(iv)
Alkyl group:
Corresponding alcohol:
Name of alcohol: 3-Methylbutan-1-ol
(v)
Alkyl group:
Corresponding alcohol:
Name of alcohol: 3-Methylbutan-2-ol
(vi)
Alkyl group:
Corresponding alcohol:
Name of alcohol: 2-Methylbutan-2-ol
(vii)
Alkyl group:
Corresponding alcohol:
Name of alcohol: 2-Methylbutan-1-ol
- From neopentane (2,2-dimethylpropane) skeleton:
(viii)
Alkyl group:
Corresponding alcohol:
Name of alcohol: 2,2-Dimethylpropan-1-ol
Q3In-text Problems
Write IUPAC names of the following compounds :
(i)
(ii)
(iii)
tetra - tert-butylmethane
Solution
To Find: IUPAC names of the given compounds.
Solution:
(i)
- First, draw the expanded structure:
- Identify the longest carbon chain. It is a 5-carbon chain (pentane). (Incorrect numbering) Numbering from either end gives the same locants (2,2,4,4).
- Identify the substituents: There are four methyl groups at positions 2, 2, 4, and 4. IUPAC Name: 2,2,4,4-Tetramethylpentane
(ii)
- First, draw the expanded structure (remember is ):
- Identify the longest carbon chain. It is a 5-carbon chain (pentane).
- Number the chain to give the substituents the lowest possible locants. Numbering from either end gives locant 3 to the methyl groups.
- Identify the substituents: There are two methyl groups at position 3. IUPAC Name: 3,3-Dimethylpentane
(iii) tetra-tert-butylmethane
- The central carbon (methane) is attached to four tert-butyl groups. A tert-butyl group is .
- Draw the expanded structure:
- Identify the longest carbon chain. The longest chain is a 5-carbon chain (pentane). Let's re-draw and find the longest chain: The main chain is pentane.
- Number the chain: Numbering from either end gives same locants.
- Identify the substituents: Two methyl groups at C-2, two methyl groups at C-4, and two tert-butyl groups at C-3. IUPAC Name: 3,3-Di-tert-butyl-2,2,4,4-tetramethylpentane
Q4In-text Problems
Write structural formulas of the following compounds :
(i)
3, 4, 4, 5-Tetramethylheptane
(ii)
2,5-Dimethyhexane
Solution
To Find: Structural formulas for the given IUPAC names.
Solution:
(i) 3, 4, 4, 5-Tetramethylheptane
- The parent alkane is heptane, which has a 7-carbon chain.
- Number the chain:
- Attach the substituents: one methyl group at C-3, two methyl groups at C-4, and one methyl group at C-5.
- Add hydrogen atoms to satisfy the valency of each carbon atom.
(ii) 2,5-Dimethylhexane
- The parent alkane is hexane, which has a 6-carbon chain.
- Number the chain:
- Attach the substituents: one methyl group at C-2 and one methyl group at C-5.
- Add hydrogen atoms to satisfy the valency of each carbon atom.
Q5In-text Problems
Write structures for each of the following compounds. Why are the given names incorrect? Write correct IUPAC names.
(i)
2-Ethylpentane
(ii)
5-Ethyl-3-methylheptane
Solution
To Find: Structures, explanation for incorrect names, and correct IUPAC names.
Solution:
(i) 2-Ethylpentane
-
Structure based on the given name: Parent chain is pentane (5 carbons). An ethyl group () is at C-2.
-
Reason for incorrect name: The name is incorrect because the longest continuous carbon chain has not been selected as the parent chain. The actual longest chain has 6 carbon atoms, not 5. (Incorrect selection) Correct longest chain:
-
Correct IUPAC Name: The longest chain is hexane. Numbering from the right end gives the methyl substituent the lowest locant (3). Correct Name: 3-Methylhexane
(ii) 5-Ethyl-3-methylheptane
-
Structure based on the given name: Parent chain is heptane (7 carbons). A methyl group is at C-3 and an ethyl group is at C-5.
-
Reason for incorrect name: The name is incorrect because the numbering of the parent chain does not follow the lowest locant rule for substituents. According to the rule, numbering should start from the end that gives the lower number to the substituent encountered first. Numbering from the left: Substituents are at 3 (methyl) and 5 (ethyl). Locants are 3, 5. Numbering from the right: Substituents are at 3 (ethyl) and 5 (methyl). Locants are 3, 5. When there is a tie in locants, alphabetical order of substituents is used to decide the numbering. 'Ethyl' comes before 'methyl', so the chain should be numbered from the right to give 'ethyl' the lower number (3).
-
Correct IUPAC Name: Numbering from the right gives ethyl group at C-3 and methyl group at C-5. Correct Name: 3-Ethyl-5-methylheptane
Q6In-text Problems
Sodium salt of which acid will be needed for the preparation of propane ? Write chemical equation for the reaction.
Solution
To Find: The carboxylic acid whose sodium salt yields propane upon decarboxylation, and the corresponding chemical equation.
Background:
The decarboxylation of a sodium salt of a carboxylic acid with soda lime () produces an alkane with one carbon atom less than the carboxylic acid.
Reaction:
Solution:
- The desired product is propane (). This alkane has 3 carbon atoms.
- The starting carboxylic acid must therefore have carbon atoms.
- A 4-carbon carboxylic acid is butanoic acid ().
- The sodium salt of this acid is sodium butanoate ().
Chemical Equation:
When sodium butanoate is heated with soda lime, it undergoes decarboxylation to form propane.
(Sodium butanoate) (Propane)
Final Answer: The sodium salt of butanoic acid is needed. The chemical equation is .
Q7In-text Problems
Write IUPAC names of the following compounds:
(i)
A structure with a ten-carbon chain, double bonds at C-3 and C-6, and methyl groups at C-2 and C-8.
(ii)
(iii)
(iv)
A structure with a ten-carbon chain, a double bond at C-4, an ethyl group at C-4, and methyl groups at C-2 and C-6.
Solution
To Find: IUPAC names for the given compounds.
Solution:
(i)
- Parent Chain: A ten-carbon chain with double bonds is a decadiene.
- Numbering: The double bonds are at C-3 and C-6. Methyl groups are at C-2 and C-8. We need to number the chain to give the double bonds the lowest possible locants. Numbering from left to right gives 3,6. Numbering from right to left gives 4,7. So, we number from left to right. The methyl groups are at C-2 and C-8.
- Name: The substituents are two methyl groups at positions 2 and 8. The parent chain is deca-3,6-diene. IUPAC Name: 2,8-Dimethyl-deca-3,6-diene
(ii)
- Parent Chain: An eight-carbon chain with four double bonds is an octatetraene.
- Numbering: The parent chain has 8 carbon atoms. The double bonds are at positions 1, 3, 5, and 7. Numbering from either end gives the same set of locants.
- Name: The positions of the double bonds are indicated before the parent name. IUPAC Name: Octa-1,3,5,7-tetraene
(iii)
- Parent Chain: The longest carbon chain containing the double bond has 5 carbon atoms (one carbon from the double bond and one of the propyl chains). So the parent alkene is pentene.
- Numbering: The numbering starts from the end that gives the double bond the lowest locant, which is 1. There is a propyl group () at position 2.
- Name: The substituent is a propyl group at C-2. IUPAC Name: 2-Propylpent-1-ene
(iv)
- Parent Chain: A ten-carbon chain with one double bond is a decene.
- Numbering: The double bond is at C-4. The substituents are an ethyl group at C-4, and methyl groups at C-2 and C-6. We must number the chain to give the double bond the lowest possible locant. Numbering from one end gives the double bond at C-4. Numbering from the other end gives it at C-6. Therefore, the first numbering scheme is correct.
- Name: The substituents are listed alphabetically (ethyl before methyl). We have an ethyl group at C-4 and two methyl groups at C-2 and C-6. IUPAC Name: 4-Ethyl-2,6-dimethyldec-4-ene