CnH2n-2d ?

1 hour ago, HbWhi5F said:

CnH2n-2d ?

Don't be silly. Think. Don't panic* and write random nonsense. What on earth does "d" mean?

First, what is the formula for alkanes? Second, think how many fewer H atoms you have for each double bond in an alkene. For instance propane CH3-CH2-CH3, vs. propene.

Tell me what you think about this.

*Actually this is tough love: I do sympathise a bit. My son used to panic with chemistry, and with maths. Stay calm and think and you will get it.

this makes me question alkane if CnH2n+2, so with 1 double bond should be (CnH2n+2)-2 removing 2 H (from adjacent C atoms) frees one 1 bond for double bond.

If there are 2 double bonds then 2x2 H are lesser so (CnH2n+2)-2x2

For 3 double bonds (CnH2n+2)-2x3

Am i right @exchemist

One day i wish i be able to pay you back for all the Knowledge and encouragement.

The formulae refer to open-chain unbranched or branched hydrocarbons. Cyclic hydrocarbons have different formulae. Each cycle (the number of cycles is defined as the number of bonds that need to be cut to form an acyclic structure¹) removes two hydrogen atoms. However, even in the case of cyclic hydrocarbons, each double bond removes two hydrogen atoms, and each triple bond removes four hydrogen atoms.

¹ Thus norbornane C₇H₁₂ is a bicyclic hydrocarbon, even though one might think it has three cycles:

On 10/26/2025 at 8:19 PM, HbWhi5F said:

(CnH2n+2)-2

i think this should be the answer but everywhere i looked it doesn't have +2

22 minutes ago, HbWhi5F said:

i think this should be the answer but everywhere i looked it doesn't have +2

For non-cyclic alkanes the 2n + 2 rule applies. For single ring cycloalkanes is it 2n, because the joining together of the ends to make a ring takes the place of what would in a straight chain be 2 H atoms. For more complicated structures like the norbornane example, it is frankly not worth memorising a rule as they won't come up often. Just draw the structure and count them!