The number of saturated and unsaturated bonds in cyclohexane are:
- (a)9 and 0 respectively.
- (b)18 and 3 respectively.
- (c)18 and 0 respectively.
- (d)9 and 3 respectively.
Correct — C, 18 and 0 respectively. Cyclohexane is C₆H₁₂, a closed ring of six carbon atoms each carrying two hydrogens. Count the bonds in two groups. Around the ring, six carbons joined end to end in a closed loop give six carbon–carbon bonds — a ring of n atoms has n bonds, not n − 1, because the last atom joins back to the first. Then each carbon holds two hydrogens, giving 6 × 2 = 12 carbon–hydrogen bonds. Six and twelve make 18 bonds in all, and because every one of them is a single bond, the count of unsaturated bonds is zero. That is what 'saturated' means: every carbon has used all four of its valencies on four separate neighbours, and there is no double or triple bond anywhere in the molecule.
- (a)9 and 0 respectively. — Right about the unsaturation and wrong about the total. Neither part of the molecule gives nine — the ring contributes six bonds and the hydrogens twelve, and the answer the question wants is their sum.
- (b)18 and 3 respectively. — The total is correct, but three double bonds have been imported from benzene. Benzene is C₆H₆ and is drawn with three double bonds around the ring; cyclohexane is C₆H₁₂, the fully saturated ring, and has none.
- (d)9 and 3 respectively. — Both errors at once — an undercounted total together with benzene's three double bonds. It is the option for someone who has confused cyclohexane with benzene and then miscounted as well.
A saturated compound is one in which every carbon–carbon link is a single bond, so each carbon is bonded to four separate neighbours and the molecule holds as much hydrogen as it possibly can. Cycloalkanes are saturated rings: cyclohexane, C₆H₁₂, is the six-membered member and is the ring most often met, since it is the framework underlying a great deal of organic chemistry. The general formula for a cycloalkane is CnH2n, which is the same formula an alkene has — a reminder that a formula alone does not settle whether a compound is saturated, and the structure must be drawn.
Counting bonds in a ring is where this item is won or lost, because the instinct carried over from chains is to count one fewer bond than there are atoms. In an open chain of six carbons there are five carbon–carbon bonds; close the chain into a ring and the sixth bond appears where the two ends join. After that the hydrogens are straightforward, two on each carbon. The other half of the question is a naming trap that is worth learning once: cyclohexane and benzene have almost the same name in a student's memory but opposite answers here, since benzene, C₆H₆, is the unsaturated one.
- Cyclohexane is C₆H₁₂ — a six-carbon ring with two hydrogens on each carbon.
- It contains 6 carbon–carbon bonds and 12 carbon–hydrogen bonds, 18 in all, every one of them single.
- A closed ring of n atoms contains n bonds between them, one more than an open chain of the same length.
- Benzene, C₆H₆, is the unsaturated six-membered ring; cyclohexane is its fully saturated counterpart.
- Cycloalkanes share the general formula CnH2n with the alkenes, so the structure and not the formula decides saturation.
A ring has as many bonds as it has atoms — the step that separates 18 from the undercounted options.
- Counting five carbon–carbon bonds in a six-carbon ring by treating it as an open chain.
- Confusing cyclohexane with benzene and importing three double bonds.
- Forgetting the carbon–hydrogen bonds and reporting only the ring skeleton.
As a bond count from a named compound, as here, or as an identification of a compound as saturated or unsaturated from its formula.
How many covalent bonds are present in a Chloropropane molecule having molecular formula, C3H7Cl ?
- (a) 6
- (b) 8
- (c) 9
- (d) 10
Answer(d) 10
The identical counting exercise on an open chain instead of a ring — two carbon–carbon bonds, seven carbon–hydrogen bonds and one carbon–chlorine bond. Comparing the two shows exactly where the extra bond in a ring comes from.
What is the total number of covalent bonds in methanol?
- (a) 3
- (b) 4
- (c) 5
- (d) 6
Answer(c) 5
The same method on a smaller molecule. Methanol gives three carbon–hydrogen bonds, one carbon–oxygen bond and one oxygen–hydrogen bond, and the discipline of listing every bond type before adding is what keeps the count honest.
- practice — not a real PYQ
The total number of covalent bonds present in one molecule of ethane, C₂H₆, is
- (a)6
- (b)7
- (c)8
- (d)12
Answer(b) 7 — one carbon–carbon bond and six carbon–hydrogen bonds, all of them single, which is what makes ethane a saturated hydrocarbon.
- practice — not a real PYQ
Which one of the following is a saturated hydrocarbon?
- (a)Ethene
- (b)Ethyne
- (c)Benzene
- (d)Cyclohexane
Answer(d) Cyclohexane — its ring is built entirely from single bonds, while ethene has a double bond, ethyne a triple bond and benzene an unsaturated aromatic ring.