Question

What order of basicity would you predict for water, methanol, isopropyl alcohol, and tert-butyl alcohol in the gas phase? Give your reasoning.

Short answer

In the gas phase, the order of basicity is: water < methanol < isopropyl alcohol < tert-butyl alcohol.

Step-by-step solution

Understand the concept of basicity

Basicity refers to the ability of a compound to accept a proton (H⁺). When assessing basicity in a series of compounds, we're looking for the molecule that can most readily accept a proton.

Consider the structure of each alcohol

Water (H₂O), methanol (CH₃OH), isopropyl alcohol ((CH₃)₂CHOH), and tert-butyl alcohol ((CH₃)₃COH) all have hydroxyl groups (-OH), but differ in their alkyl substituents. The availability of lone pair electrons on the oxygen for protonation can be influenced by sterics and inductive effects from these substituents.

Analyze steric effects

In the gas phase, bulky groups around the oxygen can hinder proton approach, reducing basicity. Water has no alkyl groups, methanol has one small methyl group, isopropyl alcohol has two larger alkyl groups, and tert-butyl alcohol has three bulky methyl groups. More steric hindrance leads to decreased basicity.

Evaluate inductive effects

The alkyl groups (methyl, isopropyl, tert-butyl) provide electron-donating inductive effects that can increase basicity by stabilizing the protonated form. Tert-butyl, having the most alkyl groups, should be most basic if considering inductive effect alone, but sterics often have a larger impact.

Predict the order of basicity

Based on the above analysis, the order in the gas phase from most to least basic should be: water < methanol < isopropyl alcohol < tert-butyl alcohol. The steric hindrance reduces the basicity for bulkier alcohols despite favorable inductive effects.

Key concepts

Basicity

Basicity is a central concept in chemistry that refers to the ability of a molecule to accept a proton, or hydrogen ion ( H^+ ). This ability is determined by the presence of lone pair electrons that are not involved in bonding and are available to form bonds with a proton.

In organic chemistry, basicity can be influenced by several factors including the structure of the molecule and the environment in which the reaction occurs. The basicity of a compound can change depending on whether it's in a gas, liquid, or solid state. This is because intermolecular forces and the availability of electrons can vary with the physical state.

For the compounds discussed here, water, methanol, isopropyl alcohol, and tert-butyl alcohol, the basicity in the gas phase is affected by their structural components and substituent groups. Recognizing the ability of these molecules to accept a proton aids in predicting their behavior in chemical reactions.

Alcohol Structure

The structure of alcohols, characterized by the presence of a hydroxyl group (-OH) attached to an alkyl or aryl group, plays a key role in determining their chemical properties such as basicity.

Water is essentially a simple alcohol with two hydrogen atoms attached to the oxygen. Methanol, with one methyl ( CH_3 ) group attached to the hydroxyl group, is a primary alcohol. Isopropyl alcohol, with two carbon-containing groups attached to the central carbon, is a secondary alcohol, while tert-butyl alcohol is a tertiary alcohol with three methyl groups attached to the bearing carbon.

As the complexity and size of the alcohol structure increase, with more extensive alkyl substitution, respective steric and inductive effects alter the basic properties. Understanding the alcohol structure helps in assessing how different groups within a molecule can influence basicity through these effects.

Steric Effects

Steric effects refer to the impact of a molecule's size and shape on its chemical reactions. In the context of alcohol basicity, the size and distribution of bulky groups around the hydroxyl (-OH) group can hinder the approach of protons, reducing the molecule's ability to act as a base.

Water has no substituents around the oxygen, making it more accessible for protonation. Methanol, with one small substituent, presents minimal steric hindrance. However, isopropyl alcohol, bearing two larger groups, introduces more hindrance. Tert-butyl alcohol's three bulky methyl groups create significant steric obstruction.

Thus, as the steric bulk increases around the hydroxyl group, the basicity tends to decrease because it becomes physically challenging for a proton to approach and attach to the oxygen atom. This demonstrates the crucial role that molecular geometry plays in determining chemical properties.

Inductive Effects

Inductive effects arise from the transmission of an electric charge through a chain of atoms in a molecule. This happens due to the presence of electron-donating or withdrawing groups, which influence the electron density around the site of reaction.

In alcohols, alkyl groups like methyl, isopropyl, and tert-butyl are electron-donating. These groups increase the electron density on the oxygen atom by releasing electrons through the sigma bonds, potentially increasing the basicity of the alcohol.

The tert-butyl group, with three alkyl groups attached, is the most electron-donating, which would imply a higher basicity if considering inductive effects alone. However, this effect can be overshadowed by steric hindrance, which limits the actual basic interaction. Understanding inductive effects helps in predicting how changes in the structure of a molecule could either increase or decrease its ability to accept protons.