Question

A hydrocarbon having only primary hydrogens is (a) Cyclohexane (b) 2-methylpropane (c) 2, 3 dimethyl but-2-ene (d) \(\mathrm{n}\) - butane

Short answer

a) Cyclohexane b) 2-methylpropane c) 2, 3 dimethyl but-2-ene d) \(\mathrm{n}\) - butane Answer: b) 2-methylpropane

Step-by-step solution

Choice (a): Cyclohexane

Cyclohexane is a ring structure containing six carbon atoms attached to form a hexagon. Each carbon atom is bonded to two hydrogen atoms. However, each carbon atom is also bonded to two other carbon atoms (in a ring), which makes it a secondary carbon, and therefore, its hydrogens are not primary.

Choice (b): 2-methylpropane

2-methylpropane has the following structure: \(\mathrm{CH_3-CH(CH_3)_2}\). Here, the central carbon atom is called a tertiary carbon because it is bonded to three other carbon atoms. The other three carbon atoms are primary carbons because they are bonded to only one other carbon atom. So, 2-methylpropane has only primary hydrogens attached to its primary carbons, making it a possible answer.

Choice (c): 2, 3 dimethyl but-2-ene

2, 3 dimethyl but-2-ene has the following structure: \(\mathrm{CH_3-C(CH_3)=CH-C(CH_3)_2}\). In this compound, the carbon atom with the double bond (C=C) is called a quaternary carbon. It is bonded to three other carbons, making it a tertiary carbon, while the other three carbons are bonded to only one other carbon atom (primary carbons). The hydrogens on the primary carbons are primary hydrogens, but the quaternary carbon has no hydrogens attached, indicating that this compound doesn't have only primary hydrogens.

Choice (d): \(\mathrm{n}\) - butane

\(\mathrm{n}\) -butane has the following structure: \(\mathrm{CH_3-CH_2-CH_2-CH_3}\). The carbon atoms at the ends (first and fourth positions) are attached to only one other carbon atom, making them primary carbons, while the other two carbon atoms in the middle are attached to two other carbon atoms making them secondary carbons. The two primary carbons have three primary hydrogens attached to them while the two secondary carbons have two secondary hydrogens attached to them. Thus, this compound doesn't have only primary hydrogens. Based on all provided choices, the only compound that has only primary hydrogens is: 2-methylpropane (choice b).

Key concepts

Hydrocarbons

Hydrocarbons form the core backbone in organic chemistry. They are compounds composed entirely of hydrogen and carbon atoms. Depending on the configuration of these atoms, hydrocarbons can be saturated, containing only single bonds, or unsaturated, having double or triple bonds. This family of compounds is further divided into alkanes, alkenes, and alkynes, each with distinctive properties. Alkanes, for instance, are known as saturated hydrocarbons due to their single carbon-carbon bonds, forming chains of varying lengths.

Hydrocarbons are fundamental in various chemical reactions and industrial applications. They serve as fuels, lubricants, and even as starting materials for creating polymers. The simplest hydrocarbons are known as the alkanes, following the formula \(C_nH_{2n+2}\), where \(n\) is the number of carbon atoms. Understanding these basic structures provides a solid foundation for more complex organic chemistry concepts.

Primary Hydrogens

In organic chemistry, primary hydrogens are those attached to a primary carbon atom. A primary carbon is one that is bonded to just one other carbon atom or none, usually found at the end of a carbon chain. These hydrogens are significant because they often participate in reactions, making them crucial in synthesis processes.

Consider the example of \(n\)-butane. At each end of its chain, the terminal carbon atoms are primary, and thus the hydrogens attached to these carbons are primary hydrogens. These primary hydrogens are more reactive in certain reactions compared to secondary or tertiary hydrogens. Recognizing primary hydrogens helps in identifying reaction sites and understanding reaction mechanisms in organic synthesis.

Carbon Atom Classification

Carbon atom classification is a key concept in understanding the structure and reactivity of organic molecules. Carbons are classified as primary, secondary, tertiary, or quaternary, depending on how many other carbon atoms they are bonded to.

• Primary carbon: Attached to one other carbon atom or none. Found at the ends of chains, it is bonded to three hydrogens in alkanes.
• Secondary carbon: Bonded to two other carbon atoms. Typically found in the middle of a chain, with two hydrogens in alkanes.
• Tertiary carbon: Connected to three other carbons, having one hydrogen if in an alkane.
• Quaternary carbon: Bonded to four carbons, bearing no hydrogens.

Understanding this classification helps predict chemical behavior. For instance, primary carbons are generally more accessible for bond formation and cleavage in reactions. This classification is a vital aspect of organic chemistry, guiding the prediction of molecular properties and reactions.

Molecular Structure of Alkanes

The molecular structure of alkanes is an important topic in organic chemistry. Alkanes are saturated hydrocarbons meaning all their carbon-carbon bonds are single. This leads to a tetrahedral geometry around each carbon atom, resulting in a relatively stable structure.

Alkanes have a general formula of \(C_nH_{2n+2}\), which reflects their saturated nature. The simplest alkane is methane, \(CH_4\), followed by ethane, propane, and butane, each incrementing the chain by one carbon and two hydrogens. These structures can be linear or branched, giving rise to isomers with different physical and chemical properties.

The molecular structure of alkanes influences properties such as boiling and melting points, where longer chains generally have higher boiling points due to increased van der Waals forces. Understanding the molecular structure aids in grasping complex topics like conformational analysis and stereochemistry, which are essential in advanced organic chemistry.