Question

The alkane which contains only primary hydrogen atom is (1) pentane (2) isopentane (3) neopentane (4) 2,2 -dimethyl butane

Short answer

Neopentane (3) is the alkane that contains only primary hydrogen atoms.

Step-by-step solution

Understand Primary Hydrogen Atoms

Primary hydrogen atoms are hydrogen atoms attached to a primary carbon atom, which in turn is bonded to only one other carbon atom.

Analyze Pentane

Pentane has the chemical formula C5H12, with a straight-chain structure. It contains primary hydrogen atoms on three carbon atoms at both ends and secondary hydrogens on middle carbons.

Analyze Isopentane

Isopentane (or 2-methylbutane) has the formula C5H12 with a branched structure. It has primary hydrogen atoms on the carbon atoms at the ends.

Analyze Neopentane

Neopentane (or 2,2-dimethylpropane) has the formula C5H12 with a structure where the central carbon atom is bonded to four other carbon atoms and no hydrogens. All hydrogen atoms are primary hydrogens because all outer carbon atoms are bonded to only one carbon atom.

Analyze 2,2-Dimethylbutane

2,2-Dimethylbutane has the formula C6H14 with a branched structure. It contains primary hydrogen atoms at the ends of the branches and secondary hydrogen atoms on the central carbons.

Conclusion

Based on the analysis, neopentane (2,2-dimethylpropane) is the only alkane that contains only primary hydrogen atoms, as all other structures include secondary and tertiary hydrogen atoms.

Key concepts

Primary Hydrogen Atoms

Primary hydrogen atoms are a crucial concept in organic chemistry. These hydrogen atoms are bonded to primary carbon atoms, which means the carbon atom is connected to only one other carbon atom.
This implies that a primary hydrogen is found on the outermost parts of the carbon chain. To identify these, look for hydrogens attached to carbons with only a single carbon neighbor.
In simpler terms, if a carbon has only one other carbon connected to it, the hydrogen atoms attached to this carbon are considered primary. Primary hydrogens significantly affect the chemical properties and reactivity of the molecule.

Alkane Structure

Alkanes are the simplest hydrocarbons consisting entirely of single bonds and following the general chemical formula CnH2n+2. The structure of alkanes can be straight-chain or branched.
In a straight-chain alkane, carbon atoms are connected in a linear sequence. For example, pentane (C5H12) is a straight-chain alkane.
In branched alkanes, some carbon atoms are connected to more than two other carbon atoms. An example of branched alkanes is neopentane (C5H12), where a central carbon is bonded to four other carbons, creating a more compact structure.
The structure influences not only the nomenclature but also the physical properties and reactivity of the alkane.

Isomer Analysis

Isomers are compounds that have the same molecular formula but different structural arrangements of atoms. Analyzing isomers is essential for understanding the diversity of organic molecules.
Consider pentane and its isomers, isopentane, and neopentane, which all have the formula C5H12. Despite having the same formula, these compounds differ in structure:

• Pentane is a straight chain without branches.
• Isopentane has a single branch off the main chain.
• Neopentane is highly branched with a central carbon atom connected to four other carbons.

This variation leads to different properties and chemical behaviors for each isomer. Isomers are pivotal in organic chemistry because they can alter a molecule's reactivity, boiling/melting points, and other physical properties.

Chemical Bonding

In alkanes, the type of bonding is predominantly single covalent bonds. Each carbon atom forms four single covalent bonds, ensuring that each connection is singular and shares one pair of electrons.
This bonding type is referred to as a sigma (σ) bond. Sigma bonds allow free rotation around the bond axis, which gives alkanes a high degree of flexibility.
The saturation of alkanes means they do not contain any double or triple bonds, distinguishing them from alkenes and alkynes, respectively. This full saturation typically makes alkanes less reactive compared to other hydrocarbons with multiple bonds.
However, their single and straightforward bonding structure contributes significantly to their stability and widespread use as fuels and lubricants.