Question

What is the molecular formula for each of the following alkynes? (a) \(\mathrm{C}_{11} \mathrm{H}_{?}\) (b) \(\mathrm{C}_{12} \mathrm{H}_{?}\)

Short answer

(a) \(\text{C}_{11}\text{H}_{20}\), (b) \(\text{C}_{12}\text{H}_{22}\)

Step-by-step solution

Understanding Alkyne General Formula

Alkynes are hydrocarbons with a triple bond and have a general formula of \( ext{C}_n ext{H}_{2n-2}\). This indicates that an alkyne has two hydrogen atoms less than the corresponding alkane.

Substituting Values for C11 Alkyne

For (a), we know the formula is \(\text{C}_{11}\text{H}_x\). We substitute \(n=11\) into the general formula \(\text{C}_n\text{H}_{2n-2}\) to get the number of hydrogens: \(x = 2(11) - 2 = 20\). Thus, the molecular formula is \(\text{C}_{11}\text{H}_{20}\).

Substituting Values for C12 Alkyne

For (b), use the same approach. Substitute \(n=12\) into \(\text{C}_n\text{H}_{2n-2}\) to find the number of hydrogens: \(x = 2(12) - 2 = 22\). Therefore, the molecular formula is \(\text{C}_{12}\text{H}_{22}\).

Key concepts

Molecular formula

A molecular formula represents the numbers and types of atoms in a molecule. It gives a concise way to show the elements involved and their respective quantities. For alkynes, the molecular formula helps identify the number of carbon (\(C\)) and hydrogen (\(H\)) atoms present. The general formula for alkynes is \(\text{C}_n\text{H}_{2n-2}\). This reflects the presence of a triple bond leading to fewer hydrogen atoms compared to their alkane counterparts, which follow the formula \(\text{C}_n\text{H}_{2n+2}\).
Understanding the molecular formula is crucial in predicting the structure and chemical behavior of hydrocarbons. For example, in the original exercise, by substituting the number of carbon atoms (\(n\)) into the formula, we can calculate the exact number of hydrogen atoms in the molecule. This process enables chemists to deduce the composition and characteristics of the molecule simply by knowing the molecular formula.

Hydrocarbons

Hydrocarbons are organic compounds composed entirely of carbon and hydrogen atoms. They form the basis of organic chemistry and are classified based on the types of bonds between carbon atoms. Alkynes are a class of hydrocarbons that contain at least one triple bond between two carbon atoms.
Alkynes, along with alkanes and alkenes, form a part of the hydrocarbon family. Each type exhibits different saturation levels and bonding structures:

• Alkanes: Saturated hydrocarbons containing only single bonds (\(\text{C}_n\text{H}_{2n+2}\)).
• Alkenes: Unsaturated hydrocarbons with at least one double bond (\(\text{C}_n\text{H}_{2n}\)).
• Alkynes: Unsaturated hydrocarbons with at least one triple bond (\(\text{C}_n\text{H}_{2n-2}\)).

Alkynes possess unique chemical properties due to their triple bond. This bond gives them a linear structure and influences their reactivity and physical properties. Understanding these differences is vital when studying organic compounds and their reactions.

Triple bond

The triple bond is a defining feature of alkynes and contributes significantly to their chemical properties. In this type of covalent bond, three pairs of electrons are shared between two carbon atoms. This makes it much stronger and shorter than single (\(\sigma\)) and double (\(\pi\)) bonds.
A triple bond consists of one sigma bond and two pi bonds. The sigma bond forms by the direct overlap of atomic orbitals, providing primary bonding between carbon atoms. The two pi bonds occur from the side-by-side overlap of p-orbitals, adding extra strength and rigidity to the bond.

• Due to the triple bond, alkynes are more reactive than alkanes and alkenes.
• They exhibit characteristic linear geometry, resulting in unique physical properties such as boiling points.
• Reactivity includes electrophilic addition and hydrogenation reactions.

Understanding the significance of the triple bond helps predict the reactivity patterns of alkynes, their stability, and the type of reactions they undergo in chemical processes.