Chapter 4 · Question 5
Distinguish between saturated and unsaturated carbon compounds with the help of examples — ethane (), ethene (), and ethyne (). Draw their electron dot structures. Which type is more reactive and why?
Q5
Distinguish between saturated and unsaturated carbon compounds with the help of examples — ethane (), ethene (), and ethyne (). Draw their electron dot structures. Which type is more reactive and why?
Answer Revealed
Direct Answer:
Saturated compounds contain only single bonds between carbon atoms. Example: ethane () — each carbon is linked by a single bond with remaining valencies satisfied by hydrogen. Electron dot structure shows one shared electron pair between carbon atoms and one shared pair between each carbon and three hydrogens. Unsaturated compounds contain double or triple bonds. Examples: ethene () has a double bond; ethyne () has a triple bond. Unsaturated compounds are more reactive because the double or triple bonds can break and add atoms without losing carbon atoms — the bonds in the multiple bonds are weaker and more accessible to attacking reagents.
Simple Explanation
In saturated compounds like ethane, all carbon–carbon bonds are single — each carbon just holds hands with its neighbour once. In unsaturated compounds, carbons hold hands twice (ethene, double bond) or three times (ethyne, triple bond). The extra bonds in unsaturated compounds are weaker and eager to break open to grab new atoms — that is what makes them more reactive. Think of it like a door — a single lock is content, but double-locked doors have a second lock that can be opened by anyone with the right key.
Exam-Ready Structure
The classification of carbon compounds as saturated or unsaturated is based on the types of covalent bonds between carbon atoms: 1. Saturated carbon compounds: (a) Definition — compounds in which all carbon atoms are linked only by single covalent bonds. (b) Example — ethane (). Step-wise construction: link two carbon atoms by a single bond (), then attach three hydrogen atoms to each carbon to satisfy the remaining valencies. Structural formula shows two carbons connected by a single bond, each bonded to three hydrogens. (c) Properties — saturated compounds (alkanes) are generally less reactive because all valence electrons of carbon are already involved in strong (sigma) single bonds. 2. Unsaturated carbon compounds: (a) Definition — compounds containing at least one double or triple bond between carbon atoms. (b) Example with double bond — ethene (): Link two carbon atoms with a single bond, attach two hydrogens to each, then form a second bond between the carbons, giving a double bond. (c) Example with triple bond — ethyne (): Link two carbons, attach one hydrogen to each, then form two more bonds, giving . 3. Electron dot structures: (a) Ethane — one shared electron pair between carbons, three shared pairs per C–H bond. (b) Ethene — two shared electron pairs between carbons (double bond), two shared pairs per C–H bond. (c) Ethyne — three shared electron pairs between carbons (triple bond), one shared pair per C–H bond. 4. Reactivity: Unsaturated compounds are more reactive than saturated ones. The double or triple bonds contain (pi) bonds in addition to the bond. The bonds are relatively weaker and can open up to add atoms (like hydrogen in addition reactions) without disrupting the carbon skeleton. This makes them chemically more versatile.
Key Points
- Saturated compounds: only single bonds between carbon atoms (e.g., ethane , methane )
- Unsaturated compounds: contain double bonds (ethene ) or triple bonds (ethyne )
- Ethane: six C–H single bonds + one C–C single bond (7 covalent bonds total)
- Ethene: four C–H single bonds + one C=C double bond
- Ethyne: two C–H single bonds + one C≡C triple bond
- Unsaturated compounds are more reactive — the bonds in double/triple bonds are weaker and open to addition
Related Questions
Q2
Draw the electron dot structures for the following molecules and indicate the type of covalent bond (single, double, or triple) formed in each: (a) hydrogen molecule (), (b) oxygen molecule (), (c) nitrogen molecule (), and (d) carbon dioxide ().
Q6
What are structural isomers? Using the molecular formula , draw the two possible structural isomers. How do chains, branches, and rings expand the diversity of carbon compounds?
Q12