Question

Name and draw structural formulas for the eight constitutional isomeric alcohols with molecular formula \(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}\). Classify each alcohol as primary, secondary, or tertiary. Which are chiral?

Short answer

Based on the given molecular formula C5H12O, eight constitutional isomeric alcohols were identified with different carbon backbone arrangements and classified as primary, secondary, or tertiary alcohols. Among these isomers, the chiral alcohols are 2-pentanol (B) and 3-methyl-2-butanol (F). These two isomers have a carbon atom with four different substituents, making them optically active.

Step-by-step solution

Identify the possible carbon structures

To form constitutional isomers with a molecular formula of C5H12O, we will need different arrangements of the carbon backbone. The possible structures are: 1. A linear chain of five carbon atoms 2. A four-carbon chain with a branching one-carbon group

Draw the isomers

Now we'll draw all the constitutional isomeric alcohols for the identified structures: 1. Linear chain of five carbon atoms: A. 1-pentanol: \(\mathrm{CH_3CH_2CH_2CH_2CH_2OH}\) (primary alcohol) B. 2-pentanol: \(\mathrm{CH_3CH_2CH(OH)CH_2CH_3}\) (secondary alcohol) 2. Four-carbon chain with branching one-carbon group: C. 2-methyl-1-butanol: \(\mathrm{CH_3CH_2CH(CH_3)CH_2OH}\) (primary alcohol) D. 2-methyl-2-butanol: \(\mathrm{CH_3CH_2C(OH)(CH_3)CH_3}\) (tertiary alcohol) E. 3-methyl-1-butanol: \(\mathrm{CH_3CH(CH_3)CH_2CH_2OH}\) (primary alcohol) F. 3-methyl-2-butanol: \(\mathrm{CH_3C(OH)(CH_3)CH_2CH_3}\) (secondary alcohol) G. 3,3-dimethyl-1-propanol: \(\mathrm{CH_3C(CH_3)_2CH_2OH}\) (primary alcohol) H. 3,3-dimethyl-2-propanol: \(\mathrm{CH_3C(OH)(CH_3)_2CH_3}\) (tertiary alcohol)

Identify chiral isomers

Chiral molecules have a carbon atom with four different substituents, which makes them optically active. Examining the eight isomeric structures above, we can identify the following chiral alcohols: B. 2-pentanol F. 3-methyl-2-butanol

Key concepts

Chiral Molecules

When you come across the term 'chiral molecules' in organic chemistry, you’re talking about a unique type of symmetry—or rather, the lack of it. Chiral molecules have a special asymmetry where they are not superimposable on their mirror image. This property is much like your hands; no matter how you rotate them, your left hand can’t be superimposed onto your right hand.

A molecule is considered chiral if it has a carbon atom attached to four different groups or substituents, also known as a stereocenter. This central carbon is called a chiral center or stereogenic center. The existence of these chiral centers leads to two forms of a molecule, which are mirror images of each other and are called enantiomers. Enantiomers have identical physical properties except for the way they interact with plane-polarized light; they rotate it in opposite directions. This phenomenon is known as optical activity.

Chiral molecules play a significant role in fields such as pharmaceuticals and biochemistry because the chirality of a molecule can affect its biological function. For the C5H12O alcohols, we identify 2-pentanol and 3-methyl-2-butanol as chiral since they have a carbon with four different groups attached to it.

Primary, Secondary and Tertiary Alcohols

Alcohols are organic compounds that feature one or more hydroxyl (-OH) groups attached to a carbon atom. But not all alcohols are created equally. They can be categorized into primary, secondary, and tertiary, based on the carbon to which the hydroxyl group is attached.

A primary (1°) alcohol has the hydroxyl group connected to a carbon atom that is attached to only one other carbon. A classic example is ethanol, where the hydroxyl group is linked to an end carbon in the chain. In our exercise, 1-pentanol and the 3-methyl-1-butanol are primary alcohols.

Secondary (2°) alcohols have their hydroxyl group on a carbon atom that is connected to two other carbons. An everyday secondary alcohol is isopropanol. 2-pentanol from the exercise falls into this category, as does the 3-methyl-2-butanol.

Tertiary (3°) alcohols feature a hydroxyl group on a carbon atom bonded to three other carbons. A well-known tertiary alcohol is tert-butanol. In the provided exercise solutions, 2-methyl-2-butanol and 3,3-dimethyl-2-propanol are tertiary. This classification is crucial because the type of alcohol affects its chemical behavior and reactivity.

Structural Formulas in Organic Chemistry

Structural formulas play an essential role in the understanding of organic chemistry. They provide a visual representation of the molecular structure of an organic compound, showing how atoms are arranged and bonded together. A structural formula includes not only the connections between atoms but also the layout of these atoms in space.

When crafting structural formulas for compounds like alcohols, chemists use lines to represent covalent bonds and letter symbols to indicate the types of atoms. A single line corresponds to a single bond, two lines to a double bond, and so on. For branched compounds, the formula branches off from the main chain to show the connection of different groups or atoms.

In our textbook problem, we’re dealing with constitutional isomers, meaning compounds that have the same molecular formula but differ in the sequence in which the atoms are connected. The precise arrangement of these atoms can lead to different properties and reactions. That’s why it’s key to understand the structural formulas — they tell us how each variant of the molecules looks and behaves differently. The exercise demonstrates this by exploring all the possible arrangements of carbon atoms for alcohols with the formula C5H12O, which has quite a variety of structures with different functional groups.