Problem 31 - 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 Type of Reaction for (a)

The reaction involves propylene, or propene (\(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\)), and bromine (\(\mathrm{Br}_{2}\)). This type of reaction is a halogenation, where bromine adds across the double bond.

Step 2: Determine the Reaction Product for (a)

In an addition reaction of bromine to a double bond, each carbon atom in the double bond will form a bond with one of the bromine atoms. So, the product is dibromopropane (\(\mathrm{CH}_3\mathrm{CHBrCH}_2\mathrm{Br}\)).

Step 3: Provide the Systematic Name for the Product of (a)

For \(\mathrm{CH}_3\mathrm{CHBrCH}_2\mathrm{Br}\), the correct systematic name is 1,2-dibromopropane.

Step 4: Identify the Type of Reaction for (b)

The reaction involves 2-pentene (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}=\mathrm{CHCH}_3\)) and hydrogen (\(\mathrm{H}_2\)). This is a hydrogenation reaction, where hydrogen adds across the double bond, converting it into a single bond.

Step 5: Determine the Reaction Product for (b)

For the hydrogenation of 2-pentene, the double bond is broken, and one hydrogen atom bonds with each carbon atom originally involved in the double bond. The result is pentane \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3\).

Step 6: Provide the Systematic Name for the Product of (b)

For the resultant \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3\), the systematic name is simply pentane, as it is a straight-chain alkane with five carbon atoms.

Key Concepts

Halogenation

Halogenation is a type of reaction in organic chemistry where a halogen atom is introduced into a compound. This process typically involves the addition of halogen molecules, like chlorine or bromine, to an unsaturated compound.

• An unsaturated compound has at least one double or triple bond.
• Alkenes, which contain double bonds, are common subjects for halogenation.

In the halogenation of alkenes, the double bond breaks, and each carbon atom in the bond connects with individual halogen atoms. For instance, when propene (\(\mathrm{CH}_3\mathrm{CH}=\mathrm{CH}_2\)) reacts with bromine (\(\mathrm{Br}_2\)), each carbon at the double bond forms a single bond with a bromine atom, resulting in dibromopropane (\(\mathrm{CH}_3\mathrm{CHBrCH}_2\mathrm{Br}\)).
This addition gives a clear example of how reactants transform under halogenation.

Hydrogenation

Hydrogenation is a significant reaction in organic chemistry that involves the addition of hydrogen to compounds. Alkenes are frequently involved in hydrogenation reactions as they contain carbon-carbon double bonds. This process turns unsaturated compounds into saturated ones by converting these double bonds into single bonds.

• In hydrogenation, hydrogen atoms are added across the double bond.
• This typically requires a catalyst, such as palladium or platinum.

For example, in the hydrogenation of 2-pentene (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}=\mathrm{CHCH}_3\)), hydrogen (\(\mathrm{H}_2\)) bonds with the carbon atoms of the original double bond. This results in the formation of pentane (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3\)), a fully saturated alkane.

Alkenes

Alkenes are a class of hydrocarbons that feature at least one carbon-carbon double bond. This double bond is pivotal in defining the chemical behavior and reactivity of alkenes.

• The general formula for alkenes is \(\mathrm{C}_n\mathrm{H}_{2n}\).
• The simplest alkene is ethene (\(\mathrm{C}_2\mathrm{H}_4\)).

This class of hydrocarbons is particularly reactive due to the presence of the double bond, which allows them to undergo addition reactions like hydrohalogenation, halogenation, and hydrogenation. These reactions typically convert alkenes into more saturated varieties, as seen with the transformation of 2-pentene to pentane, or propene to dibromopropane.
Understanding alkene chemistry is essential, as it forms the basis for various industrial processes and the synthesis of complex organic molecules.

Systematic Naming

Systematic naming in organic chemistry, governed by IUPAC rules, allows scientists and chemists to proficiently communicate about compounds. IUPAC names provide detailed structural insight into a compound’s makeup.

• Naming involves identifying the longest carbon chain to determine the base compound name.
• Functional groups and substituents attached are specified by prefixes or suffixes.

In halogenated alkanes like 1,2-dibromopropane, numbers indicate the carbon atom positions where bromines are attached. The name starts with the lowest possible numbers. Similarly, in alkanes, as with pentane (\(\mathrm{C}_5\mathrm{H}_{12}\)), the name simply follows the longest continuous carbon chain that has no substituents.
Mastering systematic naming ensures correct identification and understanding of each compound’s structure and reactivity.