Why does a 4 carbon linear alkane, butane C4H10, have two more hydrogens ?
The Correct Answer and Explanation is:
Butane, a linear alkane with the chemical formula C₄H₁₀, has two more hydrogen atoms than a hypothetical linear alkane with three carbon atoms (C₃H₈). The reason for this difference lies in the molecular structure of alkanes, which are saturated hydrocarbons composed only of carbon (C) and hydrogen (H) atoms.
Structure of Alkanes
- Saturation: Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. Each carbon atom can form four covalent bonds. In a linear alkane, each carbon is bonded to two neighboring carbons (except for the terminal carbons, which are bonded to only one neighboring carbon). The remaining bonds are filled with hydrogen atoms.
- Valency of Carbon: Carbon has a valency of four, which means it can form four bonds. In the case of butane (C₄H₁₀), with four carbon atoms in a linear arrangement, the structure can be visualized as follows:
- The two terminal carbons (C1 and C4) are each bonded to three hydrogen atoms.
- The two central carbons (C2 and C3) are each bonded to two hydrogen atoms. Thus, the total number of hydrogen atoms in butane can be calculated as follows:
- C1: 3 H
- C2: 2 H
- C3: 2 H
- C4: 3 H This results in a total of (3 + 2 + 2 + 3 = 10) hydrogen atoms.
- Comparison with Propane: For propane (C₃H₈), which has three carbon atoms:
- The terminal carbons (C1 and C3) bond to three hydrogen atoms each, while the middle carbon (C2) bonds to two hydrogen atoms.
- The total number of hydrogen atoms for propane is (3 + 2 + 3 = 8).
Conclusion
In summary, butane (C₄H₁₀) contains two more hydrogen atoms than propane (C₃H₈) due to the increased number of carbon atoms, allowing for more hydrogen bonds to satisfy carbon’s tetravalency. This highlights the structural characteristics of linear alkanes and their saturation, reinforcing the general formula for alkanes, which is (C_nH_{2n+2}). Thus, for butane, when (n = 4), the formula yields (H_{2(4)+2} = H_{10}), confirming the molecular formula C₄H₁₀.