Which of the following hydrocarbons has a double bond in its carbon skeleton? 1) c3h82) c2h63) ch44) c2h45) c2h2

C₂H₄  (option number 4)

Explanation:

A hydrocarbon with a double bond in its carbon skeleton is an alkene and has the general form:

.

This is, the number of hydrogen atoms is twice the number of carbon atoms.

On the other hand, alkanes have only single bonds, and the compounds with a triple bond in its carbon skeleton are alkynes.

Review each choice:

1) C₃H₈:

In this case, the number of hydrogen atoms is 2×3 + 2 = 6 + 2 = 8, which is corresponds to an alkane, not an alkene.

2) C₂H₆

For this, the number of hydrogen atoms is 2 × 2 + 2 = 4 + 2 = 6. Again an alkane, not alkene.

3) CH₄

Hydrogen atoms: 1 × 2 + 2 = 4 ⇒ an alkane

4) C₂H₄

Hydrogen atoms: 2 × 2  = 4.  This is precisely the relation for an alkene, so this is the hydrocarbon that has a double bond in its carbon skeleton.The chemical formula may be writen as CH₂ = CH₂, to show the double bond.

So, this is the correct answer.

5) C₂H₂

Hydrogen atoms: 2 × 2 - 2 = 4 - 2 = 2. This relation of carbon and hydrogen atoms corresponds to a compound with triple bond, i.e an alkyne: CH≡CH.

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