#3 is an alkene.
I don't know how you've been taught to do this but I do it this way.
CnH2n+2 = alkane
CnH2n = alkene (this is C20H40 and C5H10 as well as C3H6)
CnH2n-2 = alkyne
1. C10H18 - alkyne
2. C15H32 - alkane
3. C20H40 - alkyne
4. C5H10 - alkene
5. C3H6 - alkene
6. C7H16 - alkane
I am not sure on number 3
I don't know how you've been taught to do this but I do it this way.
CnH2n+2 = alkane
CnH2n = alkene (this is C20H40 and C5H10 as well as C3H6)
CnH2n-2 = alkyne
1. C10H18 - alkyne
2. C15H32 - alkane
3. C20H40 - alkene
4. C5H10 - alkene
5. C3H6 - alkene
6. C7H16 - alkane
Remember, alkynes have a triple bond, alkenes have a double bond, and alkanes have only single bonds.
Here are the determinations for each compound based on the given information:
1. C10H18: Since there is no multiple bond and it follows the general formula CnH2n+2, this compound is an alkane.
2. C15H32: Again, there is no multiple bond, and it follows the general formula CnH2n+2, so this compound is also an alkane.
3. C20H40: Without further information, it is impossible to determine the specific type of bond. It could potentially be an alkane, alkene, or alkyne. The molecular formula alone does not provide enough information to determine the presence or absence of a double or triple bond.
4. C5H10: Since there is one double bond, this compound is an alkene. Alkenes have one carbon-carbon double bond.
5. C3H6: Similarly, with one double bond present, this compound is also an alkene.
6. C7H16: Once again, there is no double or triple bond, so this compound follows the general formula CnH2n+2 and is an alkane.
In the case of C20H40, you need to calculate the number of unsaturations (double or triple bonds) in the molecule. The formula for calculating the number of unsaturations in an alkane, alkene, or alkyne is:
Unsaturation = (2 * Number of Carbons + 2) - Number of Hydrogens
For C20H40, the number of unsaturations would be:
Unsaturation = (2 * 20 + 2) - 40 = 2
Since the number of unsaturations is 2, this indicates the presence of a double bond or two triple bonds in the compound. Therefore, C20H40 is an alkyne.
So, the correct answer for number 3 is alkyne.