Problem 23 - Introductory Chemistry Atoms First

Question

Consider this incomplete drawing of a threecarbon hydrocarbon: (a) Complete the drawing by adding hydrogen atoms. (b) Complete the drawing by changing one \(C-C\) single bond to \(a C=C\) double bond and then adding hydrogen atoms. (c) Complete the drawing by changing one \(C-C\) single bond to \(a C \equiv C\) triple bond and then adding hydrogen atoms.

Step-by-Step Solution

Answer

(a) The complete drawing with added hydrogen atoms is: H-CH2-CH2-CH3 (b) The complete drawing with one C=C double bond and hydrogen atoms is: H2C=CH-CH3 (c) The complete drawing with one C≡C triple bond and hydrogen atoms is: HC≡C-CH3

Step 1: Identify bonding capacity of Carbon

Carbon forms four bonds with other atoms in a molecule, so each Carbon atom in the given hydrocarbon should have four bonds. In the given structure, one carbon has one bond, and other carbon atoms have two bonds, so, for each carbon, we should add enough hydrogens to fill the remaining bond capacity.

Step 2: Add hydrogen atoms

For the given hydrocarbon, we will add 3 hydrogen atoms to the first carbon (C1), 2 hydrogen atoms to the second carbon (C2), and 3 hydrogen atoms to the third carbon (C3). The complete drawing will be: H-CH2-CH2-CH3 #b: Change one C-C single bond to C=C double bond, and add hydrogen atoms#

Step 1: Change one C-C single bond to C=C double bond

We will change the first C-C single bond to a C=C double bond: H2C=CH-CH3

Step 2: Add hydrogen atoms where needed

Because of the double bond, the first carbon (C1) now has a bonding capacity of 4 and doesn't need any more hydrogen atoms. Carbon 2 (C2) also has a bonding capacity of 4, and doesn't need any more hydrogen atoms, but Carbon 3 (C3) still needs 3 hydrogen atoms to achieve a bonding capacity of 4. The complete drawing will be: H2C=CH-CH3 #c: Change one C-C single bond to C≡C triple bond, and add hydrogen atoms#

Step 1: Change one C-C single bond to C≡C triple bond

We will change the first C-C single bond to a C≡C triple bond: HC≡C-CH3

Step 2: Add hydrogen atoms where needed

Because of the triple bond, the first carbon (C1) now has a bonding capacity of 4 and doesn't need any more hydrogen atoms. Carbon 2 (C2) also has a bonding capacity of 4, and doesn't need any more hydrogen atoms, but Carbon 3 (C3) still needs 3 hydrogen atoms to achieve a bonding capacity of 4. The complete drawing will be: HC≡C-CH3

Key Concepts

Hydrocarbons

Hydrocarbons are fascinating compounds consisting solely of hydrogen and carbon atoms. They form the foundation of organic chemistry and are categorized based on their bonding and structure. Simplest in form, they can be either aliphatic or aromatic. Aliphatic hydrocarbons can further be divided into:

• Alkanes, which have single bonds between carbon atoms and follow the general formula of \( C_nH_{2n+2} \).
• Alkenes, which include at least one double bond between carbon atoms, utilizing the formula \( C_nH_{2n} \).
• Alkynes, which feature one or more triple bonds, adopting the formula \( C_nH_{2n-2} \).

Alkanes are saturated hydrocarbons due to having only single bonds, allowing them to bond a maximum number of hydrogen atoms. On the other hand, alkenes and alkynes are unsaturated because they can accommodate more hydrogen atoms by breaking their double or triple bonds.
Understanding the hydrocarbon structure is crucial as it impacts their physical and chemical properties like boiling points, reactivity, and energy content.

Bonding capacity of carbon

Carbon holds a unique and vital position in organic chemistry owing to its tetravalency, meaning it can form four covalent bonds with other atoms. This feature is key to the diverse and extensive nature of organic compounds.

• In organic molecules, carbon atoms can form simple linear chains, branched chains, or ring structures.
• Tetravalent nature allows for compounds of varying complexity and length because carbon can bond with itself and other atoms like hydrogen, oxygen, nitrogen, and sulfur.
• This capacity is vital for forming stable structures such as hydrocarbons, which are foundational to larger biomolecules.

A simple alkane like methane (\(CH_4\)) serves as a perfect example of carbon's tetravalence where one carbon atom forms four single bonds with hydrogen atoms. Expanding on this, complex structures like chains of carbon atoms interconnected by single or multiple bonds showcase the versatility provided by carbon's ability to bond extensively.

Double and triple bonds

In organic chemistry, the presence of double and triple bonds between carbon atoms not only determines the classification of hydrocarbons but also their reactivity and characteristics. When a carbon-carbon single bond is replaced by a double bond, it forms an alkene, while a triple bond replaces it in an alkyne.

Double bonds

A \(C=C\) double bond involves the sharing of two pairs of electrons between two carbon atoms:

• One bond is a sigma bond, which is strong and holds the atoms together.
• The second is a pi bond, which gives alkenes their unique reactivity and orientation.

This configuration makes alkenes reactive in addition reactions, where molecules like hydrogen (\(H_2\)) can be added.

Triple bonds

A \(C \equiv C\) triple bond involves sharing three pairs of electrons between carbon atoms:

• Consists of one sigma bond and two pi bonds, offering linearity and a shorter bond length than double or single bonds.
• Creates a rigid structure, making alkynes highly reactive in chemical reactions.

These multiple bonds influence a molecule's geometry and make alkynes ideal for functionalization, enhancing the creation of complex organic compounds.