Problem 22 - Chemistry The Central Science

Question

(a) Is the compound \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) saturated or unsaturated? Explain. (b) What is wrong with the formula \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{3} ?\)

Step-by-Step Solution

Answer

(a) The compound \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) is unsaturated because it contains a double bond between the second and third carbon atoms. (b) The problem with the formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{3}\) is that it shows a double bond between the third and fourth carbon atoms, but both of these carbon atoms still have two hydrogen atoms bonded to them, which is incorrect. The correct formula should be \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{2}\).

Step 1: Understand saturated and unsaturated compound

A saturated compound is a compound in which all carbon atoms are bonded to the maximum number of hydrogen atoms possible, with no double or triple bonds between carbon atoms. An unsaturated compound contains one or more double or triple bonds between carbon atoms, which results in the possibility of adding more hydrogen atoms to the compound.

Step 2: Analyse the given compound \(\mathrm{CH}_{3}\mathrm{CH}=\mathrm{CH}_{2}\)

In this formula, the carbon atoms are represented by the letter 'C', and the hydrogen atoms are represented by the letter 'H'. There is a double bond between the second and third carbon atom (indicated by the equals sign). This double bond results in lesser hydrogen atoms being bonded to the second and third carbon atoms, as seen by the subscript 1 next to both hydrogen atoms. Thus, the total hydrogen atoms in the given compound are six.

Step 3: Determine if the compound is saturated or unsaturated

As we have already identified a double bond between the second and third carbon atoms in the compound \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\), it indicates that the compound is unsaturated. An unsaturated compound contains one or more double or triple bonds between carbon atoms. Answer (a): The compound \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) is unsaturated because it contains a double bond between the second and third carbon atoms.

Step 4: Analyse the given formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{3}\)

In this formula, there are four carbon atoms and eight hydrogen atoms. However, there seems to be a double bond between the third and fourth carbon atoms. This would result in lesser hydrogen atoms being bonded to the third and fourth carbon atoms.

Step 5: Identify the issue with the formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{3}\)

The issue with the formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{3}\) is that it shows a double bond between the third and fourth carbon atoms, but both of these carbon atoms still have two hydrogen atoms bonded to them. If there were a double bond between these carbon atoms, both of them should have one hydrogen atom each. The correct formula should be \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{2}\). Answer (b): The problem with the formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}=\mathrm{CH}_{3}\) is that it shows a double bond between the third and fourth carbon atoms, but both of these carbon atoms still have two hydrogen atoms bonded to them, which is incorrect.

Key Concepts

Saturated vs Unsaturated Compounds

Organic compounds are broadly classified into saturated and unsaturated forms based on their chemical bonds. Saturated compounds are those where carbon atoms are linked by single bonds to each other, and every carbon atom is connected to the maximum possible number of hydrogen atoms. Think of them as "filled" or "complete" with hydrogens. Examples include alkanes such as methane (\(\text{CH}_4\)) and ethane (\(\text{C}_2\text{H}_6\)). Unsaturated compounds, on the other hand, contain one or more double or triple bonds. These double or triple bonds between carbon atoms imply that the compound could accept more hydrogen atoms if these multiple bonds were to break and convert into single bonds. Examples of unsaturated compounds are alkenes and alkynes like ethene (\(\text{C}_2\text{H}_4\)) and acetylene (\(\text{C}_2\text{H}_2\)). Double bonds are a clear mark of unsaturation in organic chemistry. Such compounds are not "filled" with hydrogens and may contain a reactive "pi" bond that can participate in chemical reactions, indicating the ability to take on more hydrogen atoms or undergo chemical transformations.

Structural Formulas in Organic Chemistry

A structural formula provides more information than a simple molecular formula, offering insight into how atoms are connected in a molecule. It shows the arrangement of atoms, the type of bonds between them (single, double, or triple), and sometimes, their spatial orientation. The structural formula for a compound like \(\text{CH}_3\text{CH} = \text{CH}_2\) reveals that each carbon atom has certain options for bonding. For example, the double line between the second and third carbon atoms indicates a double bond, restricting these carbons to fewer hydrogens.

• A simple molecular formula would not indicate this double bond, thus providing less information.
• Structural formulas can also be denoted using shorthand representations like condensed structures that sometimes omit hydrogen for brevity.

Understanding and interpreting structural formulas are key to grasping the detailed chemistry of a compound. They help you visualize chemical reactions and predict the behavior of compounds, especially when recognizing features like double bonds in organic molecules.

Double Bonds in Organic Compounds

Double bonds are a type of covalent bond in which two pairs of electrons are shared between two atoms, typically carbon in organic molecules. They are represented by an "equals" sign (=) in structural formulas. Each double bond consists of a sigma (\(σ\)) bond and a pi (\(π\)) bond. While the sigma bond is similar to the single bond, the pi bond is what makes double bonds unique.

• The presence of the pi bond adds rigidity and restricts the rotation around the bond axis, which is not the case with single bonds. This rigidity affects the molecule's shape and the reactions in which it can participate.
• Double bonds also characterize unsaturated hydrocarbons, providing sites for chemical reactions like hydrogenation, where hydrogen atoms are added across the double bond, converting it into a single bond and thus saturating the molecule.

Double bonds are essential for altering the chemical and physical properties of molecules, influencing factors like melting points and reactivity. Recognizing them is crucial in organic synthesis and reactions.