Problem 34 - General Chemistry: Principles and Modern Applications

Question

Give the major product that forms when 1 -ethylcyclohexene reacts with each of the following reagents: (a) \(\mathrm{HI} ;\) (b) \(\mathrm{H}_{2}\) in the presence of a platinum catalyst; (c) \(\mathrm{H}_{2} \mathrm{O}\) in \(\mathrm{H}_{2} \mathrm{SO}_{4} ;\) (d) \(\mathrm{Br}_{2}\) in \(\mathrm{CCl}_{4}\)

Step-by-Step Solution

Answer

The major products are: (a) 1-ethyl-1-iodocyclohexane, (b) ethylcyclohexane, (c) 1-ethylcyclohexanol, and (d) 1,2-dibromo-1-ethylcyclohexane.

Step 1: Reaction with Hydroiodic Acid

When HI is added to an alkene, it undergoes electrophilic addition. The hydrogen atom of HI adds to one of the carbons of the double bond and the Iodine atom to the other carbon. Resulting in the formation of 1-ethyl-1-iodocyclohexane.

Step 2: Reaction with Hydrogen Gas in the Presence of a Platinum Catalyst

Hydrogenation of alkenes with a platinum catalyst leads to the addition of hydrogen molecules across the double bond. This is a reduction reaction where two hydrogen atoms add to the two carbon atoms of the double bond in a syn addition manner. Resulting in the formation of ethylcyclohexane.

Step 3: Reaction with Water in Sulfuric Acid

Water in the presence of sulfuric acid (a strong acid) results in the formation of hydronium ions (H3O+). This acid catalyzed hydration of alkene results in the addition of a hydroxyl group (-OH) and a hydrogen atom across the double bond, with the alcohol functional group forming at the more substituted carbon atoms following Markovnikov's rule. This results in the formation of 1-ethylcyclohexanol.

Step 4: Reaction with Bromine in Carbon Tetrachloride

Bromine (Br2) in an inert solvent like carbon tetrachloride (CCl4) causes halogenation of alkenes. It is an electrophilic addition reaction where a bromine molecule adds across the double bond to form a vicinal dibromide. This yields 1,2-dibromo-1-ethylcyclohexane.

Key Concepts

Electrophilic Addition Reactions

In the wonderful world of organic chemistry, electrophilic addition reactions play a crucial role. These reactions occur when a compound with a high electron density, such as an alkene, reacts with an electrophile. An electrophile is a molecule or ion that seeks electrons, often carrying a positive charge or being polarized. Alkenes, having double bonds with high electron density, become perfect targets.

Here's a simple overview of how this addition works:

• The electrophile is attracted to the electron-rich double bond.
• One part of the electrophile attacks the double bond, disrupting it.
• This disruption causes the original double bond to break, allowing new atoms to attach to each carbon atom where the double bond was.

This type of reaction is a common way to transform alkenes into more complex molecules. One example is the addition of hydrogen iodide (HI) to 1-ethylcyclohexene. Here, the hydrogen from HI goes to one carbon, and iodine to the other, forming a new molecule: 1-ethyl-1-iodocyclohexane. It's an elegant dance of atoms, reshaping carbon frameworks through simple steps.

Hydrogenation

Hydrogenation is an exciting reaction utilized in organic chemistry to convert unsaturated compounds like alkenes into saturated ones. This involves the addition of hydrogen molecules across double bonds. Often, a metal catalyst like platinum, palladium, or nickel is used to facilitate this reaction.

Key points to remember about hydrogenation include:

• It's a reduction reaction, meaning it adds hydrogen atoms across the double bond.
• A catalyst is crucial because it guides the hydrogen to effectively attach to the carbon atoms.
• The addition of hydrogen typically occurs in a syn addition manner, where both hydrogen atoms add to the same side of the double bond.

For example, when 1-ethylcyclohexene is hydrogenated in the presence of a platinum catalyst, the double bond opens, and two hydrogen atoms attach, transforming the molecule into ethylcyclohexane. This simple addition changes the chemical properties of the molecule, making it less reactive.

Markovnikov's Rule

Markovnikov's Rule is a handy guideline in organic chemistry that helps predict the outcome of electrophilic addition reactions. The rule states that in the addition of a protic acid (like HCl, HBr, or H2O in acidic conditions) to an alkene, the hydrogen atom will bond to the carbon with the greater number of hydrogen atoms, while the other molecule bonds to the more substituted carbon.

So why does this happen?

• It's all about stability! The most stable carbocation intermediate forms when the hydrogen attaches to the less substituted carbon.
• More alkyl groups (more substituted carbon) help stabilize positive charges through hyperconjugation and induction.

In the reaction of 1-ethylcyclohexene with water in the presence of sulfuric acid, the hydroxyl group ends up on the more substituted carbon. This results in 1-ethylcyclohexanol, where the hydroxyl group forms at the carbon previously part of the double bond that has more carbon-group neighbors. By predicting products using Markovnikov’s Rule, chemists can anticipate the formation of more consistent and reliable products from reactions.

Halogenation of Alkenes

Halogenation is an interesting reaction where halogen atoms like bromine or chlorine are added to alkenes. This process, a type of electrophilic addition reaction, is particularly useful for modifying hydrocarbons, as it converts alkenes into dihalides.

In a halogenation reaction:

• The halogen molecule (like Br2) is attracted to the double bond.
• The bromine molecule interacts with the alkene's double bond, causing it to open.
• Each bromine atom then binds to one of the carbons that were part of the double bond.

When 1-ethylcyclohexene reacts with Br2 in carbon tetrachloride (CCl4), the double bond opens, and a bromine atom adds to each carbon, creating a vicinal dibromide. Consequently, 1,2-dibromo-1-ethylcyclohexane forms. This stepwise transformation through halogenation showcases the power of simple addition reactions in building more complex organic structures.