Hydrocarbons

Chemistry · 11th Class (Intermediate Part 1) · BISE Lahore

70 questions

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20 questions · Periodic Table and Periodic Properties × 1, Atomic Structure × 1, Chemical Bonding × 2, Stoichiometry × 1, States and Phases of Matter × 1, Chemical Energetics × 2, Reaction Kinetics × 1, Chemical Equilibrium × 1, Acid-Base Chemistry × 1, Electrochemistry × 2, Hydrocarbons × 2, Nitrogen and Sulfur × 1, Halogens × 1, Atmosphere × 1, Basic Separation Techniques × 1, Lab Safety and Practical Skills × 1

Hydrocarbons Test

10 questions · Hydrocarbons × 10

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Showing 10 of 70 questions from this chapter. The full bank is in the chapter test below.

1
A hydrocarbon is an organic compound that contains:
- A.only carbon and hydrogen atoms (correct answer)
- B.carbon and hydrogen together with oxygen
- C.carbon and hydrogen together with nitrogen
- D.carbon and hydrogen together with a halogen
Why: By definition a hydrocarbon contains carbon and hydrogen only. Once oxygen, nitrogen or a halogen is present, the compound is a hydrocarbon derivative such as an alcohol or a haloalkane.
2
Which of the following is an aromatic hydrocarbon?
- A.Cyclohexane
- B.Ethyne
- C.Benzene (correct answer)
- D.Cyclopropane
Why: Benzene ( $\ce{C6H6}$ ) is the basic aromatic compound on which others such as toluene are based. Cyclohexane and cyclopropane are cyclic but aliphatic, and ethyne is an acyclic unsaturated aliphatic hydrocarbon.
3
Alkanes are saturated hydrocarbons with the general formula:
- A.CnH2n-2
- B.CnH2n
- C.CnH2n+1
- D.CnH2n+2 (correct answer)
Why: An alkane contains only single bonds, so it carries the maximum number of hydrogen atoms, CnH2n+2, as in propane $\ce{C3H8}$ . CnH2n is the alkene formula and CnH2n-2 belongs to alkynes and dienes.
4
Eicosane is a straight-chain alkane whose molecules contain 20 carbon atoms. Its molecular formula is:
- A. $\ce{C20H38}$
- B. $\ce{C20H42}$ (correct answer)
- C. $\ce{C20H40}$
- D. $\ce{C20H44}$
Why: Putting n = 20 in CnH2n+2 gives $\ce{C20H42}$ . $\ce{C20H40}$ is the formula of a 20-carbon alkene or cycloalkane, which has two hydrogen atoms fewer than the corresponding alkane.
5
An alkyl group is formed when an alkane:
- A.loses one hydrogen atom (correct answer)
- B.loses one carbon atom
- C.gains one hydrogen atom
- D.loses one methyl group
Why: Removing one hydrogen atom from an alkane gives the corresponding alkyl group, whose name is obtained by changing -ane to -yl, for example $\ce{CH4}$ becomes $\ce{CH3-}$ (methyl).
6
The alkyl group ( $\ce{CH3}$ )3C- is called:
- A.n-butyl
- B.isobutyl
- C.tert-butyl (correct answer)
- D.sec-butyl
Why: The free valence is on a carbon that carries three methyl groups, so the group is the tertiary butyl (tert-butyl) group. n-Butyl is the straight chain $\ce{CH3CH2CH2CH2-}$ and isobutyl is ( $\ce{CH3}$ )2CHCH2-.
7
The parent alkane used in naming a branched-chain compound is:
- A.the chain that carries the greatest number of branches
- B.the chain written on a single line in the formula
- C.the chain with the smallest number of carbon atoms
- D.the longest continuous chain of carbon atoms (correct answer)
Why: The longest continuous carbon chain determines the parent name. That chain may be bent, so the parent name is often not obvious from the way the formula is printed.
8
In the name 4-ethyl-2-methylhexane, ethyl is written before methyl because:
- A.ethyl contains more carbon atoms than methyl
- B.substituent groups are cited in alphabetical order (correct answer)
- C.the ethyl group is attached to a lower-numbered carbon
- D.methyl groups are always placed at the end of the name
Why: Substituents are listed alphabetically, so ethyl precedes methyl. Multiplying prefixes such as di and tri are ignored while deciding this alphabetical order.
9
The longest continuous carbon chain in $\ce{CH3CH(CH3)CH2CH2CH3}$ contains:
- A.three carbon atoms
- B.four carbon atoms
- C.five carbon atoms (correct answer)
- D.six carbon atoms
Why: A continuous path that enters the branch as well as the main line contains five carbon atoms, so the compound is a methylpentane. Counting the branch as a separate carbon would wrongly give the impression of a six-carbon chain.
10
The IUPAC name of $\ce{CH3CH2CH(CH3)CH2CH3}$ is:
- A.2-ethylbutane
- B.2-methylpentane
- C.2,2-dimethylbutane
- D.3-methylpentane (correct answer)
Why: The longest chain contains five carbon atoms and the methyl branch is on the third carbon, giving 3-methylpentane. 2-Methylpentane and 2,2-dimethylbutane are different isomers of $\ce{C6H14}$ , and 2-ethylbutane is not an acceptable name because the longest chain has five carbon atoms, not four.

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