Question

What is the IUPAC name of the following compound?

A. 3,4-Dimethyl-1-pentene-3-ol
B. Isopropyl-3-methyl vinyl carbinol
C. 2,3-Dimethyl-4-pentene-3-ol
D. None of the above

Answer 1

Hint: Before finding the IUPAC name, we first need to identify the functional groups present in the compound. After identifying all of the functional groups, we need to classify them as primary and secondary and then go for the nomenclature. Also we need to find the longest carbon chain to write the root word.

Complete step by step solution:
-To find the name, first we must find the functional groups present in the given structure. Here we see that there is presence of double bond(=), an alcohol group(-OH) and a methyl group which may or may not be included in the parent chain.
-The general method of writing the IUPAC name is
Secondary prefix+primary prefix+root word+primary suffix+seconday suffix.
The word root depends on the number of carbon atoms present in the longest chain. Eg:

No. of C atoms in chain	Word root	No. of C atoms in chain	Word root
1	Meth	8	Oct
2	Eth	9	Non
3	Prop	10	Dec
4	But	11	Undec
5	pent	20	Icos
6	Hex	30	Triacont
7	Hept	100	cent

-Primary suffix includes the saturation of compounds. Single bond is alkane, double bond alkene and triple bond alkyne.
-Secondary prefix and suffix is added to indicate the functional groups. They have priorities assigned to them. Higher priority functional group is written as a suffix and all functional groups with lower priority are written as a prefix. Some of the groups can be shown as

CLASS	NAME	SUFFIX	PREFIX
R-COOH	Alkanoic acid	Oic acid	Carboxy
$R-S{O}_{3}H$	Alkane sulfonic acid	Sulfonic acid	Sulfo
R-(CO)-O-(CO)-R	Alkanoic anhydride	Oic anhydride	-----
R-COOR	Alkyl alkanoate	oate	Alkoxy carbonyl
R-(CO)-X	Alkanoyl halide	Oyl-halide	Halo Carbonyl
$R-\left(CO\right)-N{H}_2$	Alkanamide	Amide	Carbamoyl
R-CN	Alkane Nitrile	Nitrile	Cyano
R-CHO	Alkanal	-al	Oxo
R-CO-R	Alkanone	-one	oxo
R-OH	Alkanol	-ol	Hydroxy
R-SH	Alkanethiol	-thiol	mercapto
$R-N{H}_2$	alkanamide	-amine	amino

-Numbering is done such that double/triple bonds are given the highest location. Here, the location of the double bond is in the beginning itself. So numbering has to start from there only and follow till the longest carbon chain is achieved along with the minimum sum of the locants. It can be shown as

- There are 5 carbons in a chain and so the root word will be pent-
We see that 2 functional groups of methyl and alcohol are placed on the carbon at position 3 and 1 methyl group at carbon at position 4. For 2 methyl groups, we use the prefix –dimethyl and for alcohol, we use 3-ol as it is on the $3^{rd}$ carbon.
 Priority of double bond is the highest among all the functional groups and so the numbering is done from there only.

Therefore the correct name of the compound is 3,4-Dimethyl-1-pentene-3-ol and the correct option is A.

Note: Always follow all the steps of naming a compound in proper sequence. The sequence is mandatory in the naming. The priorities of different functional groups need to be memorised as it plays an important role when there are multiple number of functional groups.