Problem 23 - Chemistry and Chemical Reactivity

Question

Draw the structure and give the systematic name for the products of the following reactions: (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{Br}_{2} \rightarrow\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}+\mathrm{H}_{2} \rightarrow\)

Step-by-Step Solution

Answer

(a) 1,2-Dibromopropane; (b) Pentane.

Step 1: Identify the Reaction Type (Reaction a)

The first reaction involves the addition of bromine (\(\mathrm{Br}_{2}\)) to an alkene. This is an example of an \(\text{electrophilic} \text{addition} \text{reaction}\), where the double bond of the alkene opens up to form a dibromo compound.

Step 2: Draw the Product Structure (Reaction a)

When \(\mathrm{Br}_{2}\) is added to \(\mathrm{CH}_{3}\mathrm{CH} = \mathrm{CH}_{2}\), the double bond between the second and third carbon breaks, and each of these carbons bonds with a bromine atom. The product is \(\mathrm{CH}_{3}\mathrm{CHBrCH}_{2}\mathrm{Br}\).

Step 3: Provide Systematic Name (Reaction a)

The product from Reaction (a) is a dibromo compound. Naming groups in order of priority from left to right, the structure \(\mathrm{CH}_{3}\mathrm{CHBrCH}_{2}\mathrm{Br}\) is called \(1,2-\text{Dibromopropane}\).

Step 4: Identify the Reaction Type (Reaction b)

The second reaction involves the addition of hydrogen (\(\mathrm{H}_{2}\)) to an alkene. This is known as \(\text{hydrogenation}\), where the double bond is reduced to a single bond, resulting in an alkane.

Step 5: Draw the Product Structure (Reaction b)

For Reaction (b), \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH} = \mathrm{CHCH}_{3}\), when \(\mathrm{H}_{2}\) is added, the double bond between the third and fourth carbon becomes a single bond, and each of these carbons bonds to a hydrogen atom. The product is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\).

Step 6: Provide Systematic Name (Reaction b)

The product from Reaction (b) is a saturated hydrocarbon, i.e., an alkane. The structure \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\) is known as \(\text{pentane}\).

Key Concepts

Electrophilic Addition

Electrophilic addition is a key concept in organic chemistry, especially in reactions involving alkenes. It refers to a two-step reaction process where an electrophile reacts with a nucleophile, often resulting in the formation of a new compound. In the case of alkenes, the double bond, which is rich in electrons, acts as the nucleophile. This bond attracts the electrophile, such as a bromine molecule (\(\mathrm{Br}_{2}\)), initiating the reaction. The first step involves the attack of the electrophile on the electron-rich double bond, which forms a carbocation intermediate. In the second step, a nucleophile (often a halide ion like \(\mathrm{Br}^{-}\)) attacks the carbocation, leading to the formation of a more stable product. Electrophilic addition is an important mechanism because it explains how many organic transformations occur, particularly in the synthesis of complex molecules.

Dibromo Compounds

Dibromo compounds are a type of dihalide where two bromine atoms are added across a carbon-carbon double bond. This typically occurs during an electrophilic addition reaction involving alkenes. For instance, when bromine is added to propene (\(\mathrm{CH}_{3}\mathrm{CH} = \mathrm{CH}_{2}\)), the double bond opens, and each carbon originally involved in the double bond bonds with a bromine atom, forming \(\mathrm{CH}_{3}\mathrm{CHBrCH}_{2}\mathrm{Br}\). This new compound is called 1,2-Dibromopropane. Dibromo compounds are significant in organic synthesis and materials science due to their reactivity, serving as intermediates in the formation of more complex molecules.

Hydrogenation

Hydrogenation is a fundamental concept in organic chemistry where hydrogen gas (\(\mathrm{H}_{2}\)) is added to a molecule. In the context of alkenes, hydrogenation involves the addition of hydrogen across a carbon-carbon double bond, converting it into a single bond. This process results in the formation of an alkane from an alkene. For example, if hydrogen is added to 2-pentene (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH} = \mathrm{CHCH}_{3}\)), the double bond is reduced, forming pentane (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\)). Hydrogenation is widely used in the food industry to convert unsaturated fats into saturated fats, often known as the hardening of oils, and in the chemical industry for the synthesis of various chemicals.

Alkanes

Alkanes are the simplest form of hydrocarbons, consisting of only single bonds between carbon atoms, making them saturated. They have the general formula \(\mathrm{C}_{n}\mathrm{H}_{2n+2}\), where \(n\) is the number of carbon atoms. Alkanes are considered relatively stable due to their single covalent bonds, which makes them less reactive compared to alkenes and alkynes. In our example, pentane, which results from the hydrogenation of 2-pentene, is an alkane. Alkanes find widespread use as fuels and act as a baseline in the petrochemical industry, being part of gasoline, natural gas, and many other applications.