Alkanes, alkenes, and alkynes are the three main families of aliphatic hydrocarbons. Thanks so much.?

Question

Why does the number of hydrogen atoms in the molecular formula decreases by 2 as you go from alkanes to alkenes and then to alkyns?

Answer 1

Carbon has to have 4 bonds. (4arms reaching out)
It bonds to another carbon to make the chains.
Alkanes have all single bonds between the carbons. The other bonds on each carbon are to a H's
Alkenes have one double bond between 2 carbons. This decreases the number of arms available to reach out for an H by 2.
Alkynes have 1 triple bond between 2 carbons. This reduces the number of bonds those 2 carbons can make, they are already reaching out and holding the C near it by 3 arms.
This reducs the number of possible H's by 4


Try drawing the chains of butane, butene and butyne and see the double and triple boonding and cout the available hydrogens. Remember carbon mus have 4 bonds but can only have 4 bonds.

Answer 2

Alkanes: H3C-C3H
Alkenes: H2C=C2H
Alkynes: HC≡CH

Each carbon atom supports 4 electron-pairs, which can form covalent bonds.

In alkanes 3 of those electron-pairs go into the H-C bonds, 1 goes into the C-C bond.

In alkenes the C=C bond takes two pairs, which leaves 2 pairs for Hydrogen bonding.

In alkynes the C≡C bond uses 3 electron-pairs.

Hope this helps.

Answer 3

Each multiple bond take up the bonds which would otherwise go to two H atoms. Alkenes have a double bond - 2 less hydrogens. Alkynes have a triple bond - 4 less hydrogens.

Answer 4

Each double bond takes the place of 2 hydrogens, and a triple bond 4.

Answer 5

Because a carbon-carbon bond takes the place of a H on each carbon. (A C can form 4 bonds)