Chapter 4 · Carbon and its Compounds

NCERT Class 10 Science Chapter 4 Intext Questions (Page 68)

All 5 questions of NCERT Class 10 Science Chapter 4 Intext Questions (Page 68) — Carbon and its Compounds — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   How ​many structural ​isomers ​can you draw ​for ​pentane?
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   Three: n-pentane, 2-methylbutane and 2,2-dimethylpropane, all ${\text{C}}_5{\text{H}}_{12}$ with different carbon skeletons.

   Three structural isomers can be drawn for pentane, ${\text{C}}_5{\text{H}}_{12}$: (1) the straight chain ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$ (n-pentane); (2) a four-carbon chain with a methyl branch on the second carbon, ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_2-{\text{CH}}_3$ (2-methylbutane, isopentane); (3) a three-carbon chain with two methyl branches on the middle carbon, $\text{C}({\text{CH}}_3{)}_4$ (2,2-dimethylpropane, neopentane). All three have the same molecular formula but different carbon skeletons.

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2. Q2
   What ​are the ​two properties ​of ​carbon ​which lead to ​the huge number of carbon ​compounds we ​see around us?
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   Catenation, the ability of carbon atoms to link into chains and rings, and tetravalency, its valency of four.

   The two properties are: (1) Catenation — carbon atoms can bond with other carbon atoms to form long chains, branched chains and rings, joined by single, double or triple bonds; the carbon–carbon bond is strong and stable, so such molecules survive. (2) Tetravalency — carbon has a valency of four, so each carbon atom can bond with four other atoms of carbon or of other elements (hydrogen, oxygen, nitrogen, sulphur, chlorine and many more), giving compounds with specific properties. Because the carbon atom is small, all these bonds are strong and the compounds are exceptionally stable.

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3. Q3
   What ​will ​be the ​formula ​and electron ​dot structure of ​cyclopentane?
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   ${\text{C}}_5{\text{H}}_{10}$: a ring of five carbons, each bonded to two hydrogens, drawn with 15 shared pairs of electrons and no lone pair.

   Cyclopentane has the molecular formula ${\text{C}}_5{\text{H}}_{10}$. Its five carbon atoms form a closed ring, each carbon bonded to two ring carbons and to two hydrogen atoms. In the electron dot structure a shared pair of electrons lies between every two neighbouring carbon atoms (five C–C pairs) and between each carbon and each of its two hydrogens (ten C–H pairs) — $15$ shared pairs, $30$ electrons in all, with no lone pairs; every carbon has an octet and every hydrogen a duplet.

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4. Q4
   Draw the structures for the ​following ​compounds.

   1. (i)​Ethanoic acid
   2. (ii)Bromopentane*
   3. (iii)​Butanone
   4. (iv)​Hexanal. *Are ​structural isomers ​possible for bromopentane?
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   Structures drawn for the four compounds; bromopentane does have structural isomers (bromine on carbon 1, 2 or 3, or a branched chain).

   (i) Ethanoic acid: ${\text{CH}}_3-\text{COOH}$ — a ${\text{CH}}_3$ carbon joined to a carbon that carries a doubly bonded oxygen and an $-\text{OH}$ group. (ii) Bromopentane: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-\text{Br}$ — a five-carbon chain in which one hydrogen is replaced by bromine (drawn with Br on an end carbon), ${\text{C}}_5{\text{H}}_{11}\text{Br}$. (iii) Butanone: ${\text{CH}}_3-\text{CO}-{\text{CH}}_2-{\text{CH}}_3$ — a four-carbon chain with the ketone $\text{C}=\text{O}$ on the second carbon. (iv) Hexanal: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-\text{CHO}$ — a six-carbon chain with the aldehyde group $-\text{CHO}$ at the end. *Yes, structural isomers are possible for bromopentane: the bromine atom can be attached to carbon 1, 2 or 3 of the straight chain (1-bromopentane, 2-bromopentane, 3-bromopentane), and the carbon chain itself can be branched (for example 1-bromo-2-methylbutane); all of them have the same molecular formula ${\text{C}}_5{\text{H}}_{11}\text{Br}$.

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5. Q5
   How would you ​name the following compounds?

   1. (i)${\text{CH}}_3-{\text{CH}}_2-\text{Br}$
   2. (ii)The second structure drawn in the ​figure.
   3. (iii)​The third ​structure ​drawn ​in the figure.

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   (i) Bromoethane, (ii) methanal, (iii) hexyne (hex-1-yne).

   (i) Bromoethane — two carbons (ethane) with a bromine atom in place of one hydrogen. (ii) Methanal — one carbon carrying the aldehyde group $-\text{CHO}$ (commonly called formaldehyde). (iii) Hexyne — a six-carbon chain with a triple bond, ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-\text{C}\equiv \text{CH}$; since the triple bond is on the first carbon, it is hex-1-yne.

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