Problem 74 - General Chemistry

Question

For each of the following molecules, create the correct condensed structural formulas by adding and/ or removing hydrogen atoms (do not add or remove bonds). a \(\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}\) b \(\mathrm{CH}_{3} \equiv \mathrm{CHCH}_{2} \mathrm{CH}_{2}\) c \(\mathrm{CH}_{3} \mathrm{CHC}=\mathrm{CCH}_{2}\)

Step-by-Step Solution

Answer

a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CH}_{2}\), b) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\), c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{C}=\mathrm{CH}_{2}\)."

Step 1: Analyze the Given Structure for Molecule (a)

For the molecule \(\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}\), it's important to recognize that it initially has a double bond. Count the hydrogen atoms to ensure each carbon satisfies the valence of four bonds.

Step 2: Add Hydrogens to Unsaturated Carbon Atoms

In \(\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}\), the second carbon from the left has only one hydrogen attached and forms two bonds with its neighboring carbons. Therefore, add two more hydrogens. The third carbon has a double bond with the next carbon, requiring only one hydrogen. The condensed structure becomes \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CH}_{2}\).

Step 3: Analyze the Given Structure for Molecule (b)

The molecule \(\mathrm{CH}_{3} \equiv \mathrm{CHCH}_{2} \mathrm{CH}_{2}\) has a triple bond initially. Consider the hydrogen count for the carbons involved in triple and single bonds.

Step 4: Adjust Hydrogen to Match Triple Bond Requirements

For \(\mathrm{CH}_{3} \equiv \mathrm{CHCH}_{2} \mathrm{CH}_{2}\), ensure the second carbon (in a triple bond) has no hydrogens since it's bonded to another carbon with a triple bond already. The correct condensed formula is \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\).

Step 5: Analyze the Given Structure for Molecule (c)

In \(\mathrm{CH}_{3} \mathrm{CHC}=\mathrm{CCH}_{2}\), there's a double bond. Count hydrogens to make each carbon reach four bonds. The last carbon appears to need additional hydrogens.

Step 6: Adjust the Terminal Carbon's Hydrogens

Since the fourth carbon requires two more hydrogen bonds (total count of three), adjust the structure to \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{C}=\mathrm{CH}_{2}\). Each double-bonded carbon maintains its hydrogens to satisfy the valency rule.

Key Concepts

Hydrogen Addition

Hydrogen addition is an essential part of understanding molecular structures, especially when dealing with hydrocarbons. In chemical reactions, especially those involving unsaturated hydrocarbons like alkenes and alkynes, adding hydrogen atoms allows the conversion to a more saturated molecule.
This process involves pairing each hydrogen with a carbon atom to fulfill the carbon's preference for forming four bonds. To visualize this, consider an alkene, which contains at least one carbon-carbon double bond. By adding hydrogen to each carbon participating in the double bond, the alkene can be converted into an alkane, which has only single bonds.
Overall, hydrogen addition affects the formula and properties of the molecule, transforming its structured characteristics.

Hydrocarbon Bonding

Hydrocarbons can form various types of bonds, single, double, or triple, due to the carbon atoms' ability to bond in multiple ways. These bonds essentially determine the molecular structure and its corresponding reactivity. Single bonds (alkanes), represented as \( \text{C-C} \), are the simplest form, showing each carbon bonded to four others through single bonds.
Double bonds (alkenes), such as depicted in \( \text{C=C} \), are formed when two pairs of electrons are shared between two carbon atoms, allowing hydrogen atom addition for saturation. Triple bonds (alkynes), such as those represented by \( \text{C} \equiv \text{C} \), involve sharing three pairs of electrons between two carbons, making the molecule even less saturated and requiring no additional hydrogen on the carbons involved in the triple bond.

• Single bond: Allows free rotation and makes the molecule more flexible.
• Double bond: Restricted rotation, creates a flat structure.
• Triple bond: Linear and firm structure due to rigidity.

Understanding these bonding differences is crucial for predicting molecular behavior in reactions.

Valency of Carbon

The valency of an atom indicates the number of chemical bonds it can form. Carbon, a key element in organic chemistry, has a valency of four. This means a carbon atom forms four covalent bonds with other atoms, allowing it to build complex and diverse structures.
For example, in methane (\( \text{CH}_4 \)), each carbon is bonded to four hydrogen atoms, fulfilling its valency. In more complicated molecules, carbon can form multiple bonds with other carbons or different elements, while still adhering to the four-bond rule.

• Single-bonded carbon usually bonds with four hydrogen atoms in saturated hydrocarbons.
• Double-bonded carbon links with two other functional groups.
• Triple-bonded carbon features one other bond beyond the triple bond.

Recognizing how carbon's valency bases its ability to engage in diverse bonding patterns is fundamental for drawing accurate molecular structures.