Question

Propose skeletal structures for compounds that satisfy the following molecular formulas. There is more than one possibility in each case. (a) \(\mathrm{C}_{5} \mathrm{H}_{12}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{7} \mathrm{~N}\) (c) \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) (d) \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{Cl}\)

Short answer

Each formula represents several structural isomers due to different possible arrangements of atoms.

Step-by-step solution

Identify the Alkanes

The formula \( \mathrm{C}_{5} \mathrm{H}_{12} \) represents an alkane with five carbon atoms. Alkanes have the general formula \( \mathrm{C}_{n}\mathrm{H}_{2n+2} \). Hence, these compounds are possible structural isomers: Pentane, 2-Methylbutane (isopentane), and 2,2-Dimethylpropane (neopentane). Each has a distinct arrangement of carbon atoms.

Positive Nitrogen Compounds

The formula \( \mathrm{C}_{2} \mathrm{H}_{7} \mathrm{~N} \) represents a compound with nitrogen, typically an amine. Possible structures include Ethylamine, which is a straight chain with a structure \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2} \), and Dimethylamine, which has the structure \( \mathrm{(CH}_{3})_{2}\mathrm{NH} \). These structures represent different functional groups in organic chemistry.

Explore Oxygen-Containing Compounds

\( \mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O} \) can represent several types of compounds. It can be propanal, which has an aldehyde group and the structure \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO} \). Alternatively, it could be acetone, a ketone with the structure \( \mathrm{(CH}_{3})_{2}\mathrm{CO} \). Propene oxide is another possibility, which is an epoxide with three carbon atoms.

Include Chlorinated Compounds

\( \mathrm{C}_{4} \mathrm{H}_{9}\mathrm{Cl} \) indicates an alkyl chloride with four carbon atoms. Possible structures are 1-Chlorobutane, 2-Chlorobutane, tert-Butyl chloride (2-Chloro-2-methylpropane), and isobutyl chloride (1-Chloro-2-methylpropane). Each represents a different arrangement of carbon and chlorine.

Key concepts

Alkanes

Alkanes are a fundamental class of hydrocarbons that consist solely of carbon and hydrogen atoms. They follow a simple formula: \( \mathrm{C}_{n}\mathrm{H}_{2n+2} \). This means each carbon atom bonds with four other atoms, usually hydrogens or other carbons. This saturated nature is why alkanes are sometimes called "saturated hydrocarbons."

• Pentane, with the formula \( \mathrm{C}_{5} \mathrm{H}_{12} \), is a classic example of an alkane. It has a chain of five carbon atoms through which hydrogen atoms branch off, ensuring each carbon forms four bonds.
• Isomers such as 2-Methylbutane and 2,2-Dimethylpropane create different structures by varying the carbon chain's arrangement.

Alkanes are quite stable due to their strong C-C and C-H single bonds, making them less reactive than other hydrocarbons. However, they can undergo reactions like combustion in the presence of sufficient oxygen.

Isomers

Isomers are molecules with identical molecular formulas but different structural arrangements of atoms. This means while they contain the same number and type of atoms, their connections differ, resulting in diverse properties. There are several types of isomers, but in the context of alkanes and other simple organic compounds, structural isomers are most relevant.

• Structural (or constitutional) isomers differ in how their atoms are bonded. For example, pentane, 2-methylbutane, and 2,2-dimethylpropane all share the formula \( \mathrm{C}_{5} \mathrm{H}_{12} \) but differ in carbon chain arrangement.
• Different isomers can have significantly different physical and chemical properties, even though they have the same formula.

Understanding isomers is crucial for chemists because different isomers can react in unique ways in chemical reactions.

Functional Groups

Functional groups are specific groupings of atoms within molecules that have characteristic properties and chemical reactivities. They are the active centers in organic molecules and define how those molecules interact with others.

• Amines are a type of functional group that include nitrogen, such as in ethylamine \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2} \) and dimethylamine \( \mathrm{(CH}_{3})_{2}\mathrm{NH} \). These groups are involved in many biochemical processes.
• Oxygen-containing functional groups include aldehydes and ketones. An example of an aldehyde is propanal \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO} \), whereas acetone \( \mathrm{(CH}_{3})_{2}\mathrm{CO} \) is a ketone.
• Chlorinated compounds like 1-chlorobutane have a chlorine atom as a functional group, affecting the molecule's properties such as reactivity and polarity.

Recognizing and understanding functional groups is pivotal in predicting the behavior of organic molecules across various chemical reactions and applications.