Question

Using only isobutene and any inorganic reagents synthesize \(2,2,4\) -trimethylpentane.

Short answer

To synthesize 2,2,4-trimethylpentane from isobutene, follow these steps: 1. Hydrate isobutene with an acid catalyst to produce 2-methyl-2-propanol. 2. Dehydrate 2-methyl-2-propanol using a strong acid, forming 2-methyl-2-butene. 3. Hydrate 2-methyl-2-butene, following Markovnikov's rule, to create 2,3-dimethyl-2-butanol. 4. Replace the hydroxy group in 2,3-dimethyl-2-butanol with a halogen using hydrogen halide, yielding 2,3-dimethyl-2-bromo-butane. 5. Perform dehydrohalogenation with a strong base to form 2,2,4-trimethylpent-2-ene. 6. Catalytically hydrogenate the product using a metal catalyst, resulting in 2,2,4-trimethylpentane.

Step-by-step solution

Hydration of Isobutene

To start with, we'll need to hydrate isobutene (2-methylpropene) to produce 2-methyl-2-propanol. This is essentially just adding a water molecule across the double bond. Hydration is often carried out with an acid catalyst, typically sulfuric acid or phosphoric acid.

Acidic Dehydration

The next step is to dehydrate the 2-methyl-2-propanol. This is achieved by heating the alcohol in the presence of a strong acid, such as sulfuric acid. This causes the elimination of a water molecule and formation of a more substituted alkene, 2-methyl-2-butene.

Second Hydration

Then, hydrate the 2-methyl-2-butene to get 2,3-dimethyl-2-butanol. Here, we add water across the double bond. This is a delicate process because we want to hydrate the internal carbon of the butene, not the terminal carbon. The major product will be the one where the water adds to the more substituted carbon atom, in accordance with Markovnikov's rule.

Halogenation

Next, the hydroxy group in 2,3-dimethyl-2-butanol is replaced with a halogen using a reaction with hydrogen halide (HI or HBr) to give 2,3-dimethyl-2-bromo-butane. This is a substitution reaction where the hydroxy group is replaced by a bromine atom.

Dehydrohalogenation

The bromo group in 2,3-dimethyl-2-bromo-butane is next eliminated via dehydrohalogenation to yield 2,2,4-trimethylpent-2-ene. This is achieved by reacting the bromo-butane with a strong base (usually ethanolic KOH).

Catalytic Hydrogenation

Finally, the 2,2,4-trimethylpent-2-ene is hydrogenated to provide the target molecule, 2,2,4-trimethylpentane. This involves the geotropic absorption of H2 across the double bond in the presence of a suitable metal catalyst, such as platinum or palladium.

Key concepts

Isobutene

Isobutene, also known as 2-methylpropene, is a common hydrocarbon with a branched structure. It serves as a vital starting material in organic synthesis, particularly for producing valuable chemicals. Isobutene is colorless and emits a petroleum-like odor. It's a gas at room temperature but can be liquefied under pressure.
In industrial settings, isobutene is often produced as a by-product of refining and polymer production processes. Its structure, with a double bond and branching, makes it highly reactive and suitable for various chemical reactions, such as polymerization and synthesis of alcohols. Due to its reactivity, isobutene is particularly useful in producing compounds like 2,2,4-trimethylpentane through different organic reactions.

2-Methylpropene

2-Methylpropene is another name for isobutene, highlighting its chemical structure with a methyl group attached to the second carbon of a propene chain. This notation is crucial in organic chemistry for classifying different isomers of hydrocarbons.
Understanding its structure helps chemists predict reactions. The presence of the double bond indicates places where reactions can occur, such as additions or hydration. This characteristic is advantageous in synthetic pathways, where 2-methylpropene can act as a precursor to diverse organic chemicals, such as acids and alcohols, through mechanisms like hydration or polymerization.

Markovnikov's Rule

In organic chemistry, Markovnikov's rule is a guiding principle for predicting the outcomes of addition reactions. Specifically, it is used when adding a component like water (H-OH) across a double bond. The rule states that the hydrogen atom will usually attach to the carbon with the most hydrogen atoms, while the other part (e.g., OH) attaches to the carbon with fewer hydrogens.
This makes the more substituted product predominant. During the hydration steps in synthesis, Markovnikov's rule helps chemists predict where new bonds will form, which is especially useful in forming specific alcohols from alkenes.

Dehydrohalogenation

Dehydrohalogenation is a chemical reaction where a hydrogen halide (like HBr) is eliminated from an organic molecule. It creates alkenes from alkyl halides by removing a hydrogen and a halogen from adjacent carbon atoms. This reaction typically requires a strong base, such as potassium hydroxide (KOH).
In organic synthesis, dehydrohalogenation is employed to introduce double bonds into a compound. It serves as an essential step in transitioning from haloalkanes to alkenes, forming desired structures like 2,2,4-trimethylpent-2-ene, which can undergo further reactions like hydrogenation.

Catalytic Hydrogenation

Catalytic hydrogenation is an important process used to convert alkenes into alkanes by adding hydrogen atoms across carbon-carbon double bonds. This reaction requires a catalyst, often a metal such as platinum or palladium, to facilitate the absorption of hydrogen gas. The catalyst surface enables the breaking of H-H bonds and the subsequent addition of H atoms to the alkene.
This method is widely used in refining, food processing, and organic synthesis, allowing the conversion of unsaturated compounds into their saturated counterparts. In the final steps of organic synthesis, catalytic hydrogenation helps in stabilizing the structure, producing compounds like 2,2,4-trimethylpentane.