Question

Assign the IUPAC name to \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}\right]_{3} \mathrm{COH}\). (a) 3-(2-methylethyl)- \(3(1\)-methylethyl) pentan-2-ol (b) tris-(1-methylpropyl)methanol (c) 2,4 -dimethyl-3 (1-methylethyl) pentan- \(2-\mathrm{ol}\) (d) 3 -(1-methylethyl) \(-2,4\)-dimethylpentan-3-ol

Short answer

The IUPAC name is (b) tris-(1-methylpropyl)methanol.

Step-by-step solution

Identify the Longest Carbon Chain

The given compound is \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}\right]_{3} \mathrm{COH}\). Start by identifying the longest continuous carbon chain containing the hydroxyl (-OH) group. Count the carbons in this chain to determine its length and assign the base name. The chain has 5 carbon atoms, so it’s a pentan-ol.

Identify and Name Substituents

Observe the structure of the compound: \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}\right]_{3} \mathrm{COH}\). There are three \(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CH}\) groups attached to a central \(\mathrm{C-OH}\). These are isopropyl groups. Therefore, the substituents are 1-methylethyl groups, also known as isopropyl groups.

Determine the Base Name and Number the Chain

The longest chain has a length of 5 (pentane). The -OH group is on carbon 2 indicating 'pentan-2-ol'. We need to number the carbon chain such that the substituent groups receive the lowest possible numbers.

Assign Locants and Assemble the IUPAC Name

Detail the positions of the substituents: three 1-methylethyl groups occupy positions as trisubstituents. Hence, it is named as 'tris-(1-methylpropyl)methanol', where the central carbon is part of the base structure.

Double Check the Correct IUPAC Name Option

Cross-check with the provided options. We know that the structure represents a central carbon skeleton with three identical isopropyl groups attached to the central carbon with the hydroxyl (-OH) group. The correct systematic name is 'tris-(1-methylpropyl)methanol', aligning exactly with option (b).

Key concepts

Longest Carbon Chain Identification

When determining the IUPAC name of a compound, the first essential step is identifying the longest continuous chain of carbon atoms. This chain must include any functional groups present, such as hydroxyl (OH) groups. The length of this main carbon chain dictates the base name of the compound.

For example, in the compound \[\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}\right]_{3} \mathrm{COH}\]the longest carbon chain containing the hydroxyl group has five carbon atoms. Therefore, the base name derived from this structure is "pentan". Because the hydroxyl group is involved, it extends to "pentan-ol".

Always ensure the chosen chain provides the longest sequence and includes the highest priority functional group the compound contains. This ensures the base name correctly represents the compound's main structural feature.

Functional Group Priority

Functional groups determine the reactivity and properties of a molecule. In IUPAC nomenclature, determining their priority is crucial for accurate naming. The functional group that holds the highest priority is the one incorporated into the compound's base name.

The hydroxyl (-OH) group, which denotes alcohols, is typically among the higher priority groups. It is higher in priority than alkanes, alkenes, and most substituents such as halo groups or alkyl chains. In our example compound, the presence of a hydroxyl group on the pentane chain makes the base name "pentan-ol".

It's important to identify all functional groups in a molecule and assign numbers so that the group of highest priority is given the lowest possible number. Prioritizing functional groups properly ensures the main functional group is accurately highlighted in the molecule's name.

Substituent Naming

Substituents are groups attached to the main carbon chain and need to be named and positioned accurately. The naming usually involves identifying the substituent's type and its location on the principal chain.

In the given compound, the substituents are \[\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CH}\]groups known as isopropyl groups. Because these are derived from propane but missing one hydrogen atom, they officially get the name "1-methylethyl" in IUPAC terms. When naming these substituents, you should also use prefixes like "di-, tri-," etc., if there are multiple identical groups.

The provided compound has five carbon atoms in its main chain, and the substituents' exact positions are detailed numerically in the full IUPAC name. Always number the chain to give the substituents the lowest possible locants, thus clarifying their locations.

Hydroxyl Group Positioning

The position of the hydroxyl group is crucial for correctly naming alcohols. This group should be numbered as low as possible in the identified longest carbon chain because it is the high priority functional group in alcohols.

For instance, in the compound we are examining, the hydroxyl group is located on the second carbon of a five-carbon chain, which dictates "pentan-2-ol" in the name.

When multiple functional groups are present, the position of each should be factored in, with the hydroxyl group usually having priority. If there are other functional groups, ensure the numbering reflects the highest priority system. Proper positioning of the hydroxyl group in the IUPAC name helps to demonstrate its role and location in the molecular structure.