Basics of Organic Chemistry
Basics of Organic Chemistry MCQ Questions
1.
What is the general molecular formula of alkanes?
Approach
The question asks for the general molecular formula of alkanes. Alkanes are saturated hydrocarbons, meaning they consist only of carbon and hydrogen atoms and contain only single bonds. To determine their general formula, we can look at the simplest alkanes and identify a pattern.
Step-by-step
- Understand Alkanes: Alkanes are a homologous series of organic compounds. Each member differs from the next by a \(\text{CH}_2\) unit. They are characterized by having only single bonds between carbon atoms and between carbon and hydrogen atoms.
- Examine Simple Alkanes:
- Methane (\(\text{CH}_4\)): Here, \(n=1\), and the number of hydrogen atoms is \(2(1)+2 = 4\).
- Ethane (\(\text{C}_2\text{H}_6\)): Here, \(n=2\), and the number of hydrogen atoms is \(2(2)+2 = 6\).
- Propane (\(\text{C}_3\text{H}_8\)): Here, \(n=3\), and the number of hydrogen atoms is \(2(3)+2 = 8\).
- Butane (\(\text{C}_4\text{H}_{10}\)): Here, \(n=4\), and the number of hydrogen atoms is \(2(4)+2 = 10\).
- Identify the Pattern: From these examples, it is clear that for an alkane with \(n\) carbon atoms, the number of hydrogen atoms is always \(2n+2\).
- Formulate the General Formula: Therefore, the general molecular formula for alkanes is \(\text{C}_n\text{H}_{2n+2}\).
Correct Option:
D) \(\text{C}_n\text{H}_{2n+2}\) is the correct general molecular formula for alkanes, as derived from the structures of simple alkanes.
Incorrect Options
- A) \(\text{C}_n\text{H}_{2n-2}\) is the general molecular formula for alkynes (compounds with a carbon-carbon triple bond) or cycloalkenes with one double bond.
- B) \(\text{C}_n\text{H}_{2n}\) is the general molecular formula for alkenes (compounds with a carbon-carbon double bond) or cycloalkanes.
- C) \(\text{C}_n\text{H}_n\) does not represent a common homologous series of simple hydrocarbons. For example, benzene is \(\text{C}_6\text{H}_6\), but this formula is not general for a series like alkanes, alkenes, or alkynes.
2.
Which of the following is the simplest alkane?
Approach
The question asks for the simplest alkane. Alkanes are saturated hydrocarbons with the general formula \(C_nH_{2n+2}\). The "simplest" alkane refers to the one with the fewest carbon atoms.
Step-by-step
- Recall the definition of an alkane: a hydrocarbon with only single bonds and the general formula \(C_nH_{2n+2}\).
- To find the simplest alkane, we need to find the alkane with the smallest possible number of carbon atoms.
- The smallest possible value for \(n\) (number of carbon atoms) in an alkane is \(n=1\).
- Substitute \(n=1\) into the general formula \(C_nH_{2n+2}\):
\[ C_1H_{2(1)+2} = CH_4 \]
This compound is Methane. - Compare this with the given options to identify the correct answer.
Correct Option:
C) Methane (CH4) is the simplest alkane because it has only one carbon atom, which is the minimum number of carbon atoms an alkane can have according to the general formula \(C_nH_{2n+2}\).
Incorrect Options
- A) Butane (C4H10) — Butane has four carbon atoms, making it a larger and more complex alkane than methane, ethane, or propane.
- B) Propane (C3H8) — Propane has three carbon atoms, making it more complex than methane and ethane.
- D) Ethane (C2H6) — Ethane has two carbon atoms, making it simpler than propane and butane, but still more complex than methane.
3.
What is the hybridisation of carbon atoms in alkanes?
Approach
To determine the hybridisation of carbon atoms in alkanes, we need to understand the bonding characteristics of carbon in these compounds. Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms and between carbon and hydrogen atoms. Hybridisation is determined by the number of sigma bonds and lone pairs around the central atom.
Step-by-step
- Identify the structure of alkanes: Alkanes are organic compounds consisting entirely of single-bonded carbon and hydrogen atoms. The simplest alkane is methane (\(\text{CH}_4\)), followed by ethane (\(\text{C}_2\text{H}_6\)), propane (\(\text{C}_3\text{H}_8\)), and so on.
- Examine a carbon atom in an alkane: Consider a carbon atom in any alkane. For instance, in methane (\(\text{CH}_4\)), the central carbon atom is bonded to four hydrogen atoms. In ethane (\(\text{C}_2\text{H}_6\)), each carbon atom is bonded to three hydrogen atoms and one other carbon atom.
- Count sigma bonds and lone pairs: In an alkane, each carbon atom forms four single covalent bonds. These single bonds are always sigma (\(\sigma\)) bonds. There are no lone pairs of electrons on a carbon atom in a stable alkane structure.
- Determine the steric number: The steric number is the sum of the number of sigma bonds and the number of lone pairs around the central atom. For a carbon atom in an alkane, the steric number is \(4\) (four sigma bonds + zero lone pairs).
- Relate steric number to hybridisation:
- Steric number \(2\) corresponds to \(sp\) hybridisation.
- Steric number \(3\) corresponds to \(sp^2\) hybridisation.
- Steric number \(4\) corresponds to \(sp^3\) hybridisation.
Since the steric number for a carbon atom in an alkane is \(4\), its hybridisation is \(sp^3\). This \(sp^3\) hybridisation leads to a tetrahedral geometry around each carbon atom with bond angles of approximately \(109.5^\circ\).
Correct Option:
D) sp 3 — Carbon atoms in alkanes form four single bonds (sigma bonds) and have no lone pairs. This results in a steric number of 4, which corresponds to \(sp^3\) hybridisation.
Incorrect Options
- A) sp 2 — \(sp^2\) hybridisation occurs when a carbon atom forms one double bond and two single bonds (e.g., in alkenes). This results in a steric number of 3.
- B) sp 3 d — \(sp^3d\) hybridisation involves the participation of d-orbitals and is typically observed in elements from the third period or below, which have accessible d-orbitals (e.g., phosphorus in \(\text{PCl}_5\)). Carbon, being in the second period, does not use d-orbitals for bonding.
- C) sp — \(sp\) hybridisation occurs when a carbon atom forms one triple bond and one single bond, or two double bonds (e.g
4.
The bond angle in methane (CH4) is approximately:
Approach
To determine the bond angle in methane (CH₄), we need to consider its molecular geometry and hybridization. Methane is a simple molecule with a central carbon atom bonded to four hydrogen atoms. The VSEPR (Valence Shell Electron Pair Repulsion) theory helps predict the geometry and bond angles.
Step-by-step
- Determine the central atom and its valence electrons: In CH₄, carbon (C) is the central atom. Carbon has 4 valence electrons.
- Determine the number of atoms bonded to the central atom: There are 4 hydrogen (H) atoms bonded to the carbon atom.
- Calculate the steric number: The steric number is the sum of the number of bonded atoms and the number of lone pairs on the central atom. For CH₄, there are 4 bonded atoms and 0 lone pairs on carbon (since all 4 valence electrons are used in bonding with hydrogen). So, the steric number is \(4 + 0 = 4\).
- Determine the hybridization: A steric number of 4 corresponds to \(sp^3\) hybridization.
- Predict the molecular geometry: For a molecule with \(sp^3\) hybridization and no lone pairs, the electron geometry and molecular geometry are both tetrahedral.
- Determine the ideal bond angle for tetrahedral geometry: In an ideal tetrahedral geometry, the atoms are arranged symmetrically around the central atom, leading to a specific bond angle. This ideal bond angle is approximately \(109.5^\circ\).
Correct Option:
C) 109.5° is the characteristic bond angle for a molecule with ideal tetrahedral geometry, such as methane (CH₄).
Incorrect Options
- A) 120° — This bond angle is characteristic of trigonal planar geometry (e.g., in molecules with \(sp^2\) hybridization like BF₃ or ethene). Methane does not have this geometry.
- B) 180° — This bond angle is characteristic of linear geometry (e.g., in molecules with \(sp\) hybridization like CO₂ or ethyne). Methane does not have this geometry.
- D) 90° — While 90° angles exist in some molecular geometries (e.g., square planar or octahedral), it is not the bond angle for methane's tetrahedral structure. The atoms in a tetrahedron are spread out to minimize repulsion, resulting in a larger angle than 90°.
5.
Which of the following is NOT an alkane?
Approach
To identify which compound is NOT an alkane, we need to recall the general formula for alkanes and apply it to each given option. Alkanes are saturated hydrocarbons with the general formula \(C_nH_{2n+2}\).
Step-by-step
- Understand the definition of an alkane: Alkanes are a class of hydrocarbons that consist only of single bonds between carbon atoms and between carbon and hydrogen atoms. They are saturated compounds.
- Recall the general formula for alkanes: The general molecular formula for alkanes is \(C_nH_{2n+2}\), where \(n\) represents the number of carbon atoms.
- Apply the formula to each option:
- Option A: \(C_2H_6\). Here, \(n=2\). According to the formula, the number of hydrogen atoms should be \(2n+2 = 2(2)+2 = 4+2 = 6\). Since it matches, \(C_2H_6\) is an alkane (ethane).
- Option B: \(C_4H_{10}\). Here, \(n=4\). According to the formula, the number of hydrogen atoms should be \(2n+2 = 2(4)+2 = 8+2 = 10\). Since it matches, \(C_4H_{10}\) is an alkane (butane).
- Option C: \(CH_4\). Here, \(n=1\). According to the formula, the number of hydrogen atoms should be \(2n+2 = 2(1)+2 = 2+2 = 4\). Since it matches, \(CH_4\) is an alkane (methane).
- Option D: \(C_3H_6\). Here, \(n=3\). According to the formula, the number of hydrogen atoms should be \(2n+2 = 2(3)+2 = 6+2 = 8\). The given compound has only 6 hydrogen atoms, which does not match the alkane formula. This compound fits the general formula for alkenes, \(C_nH_{2n}\), which is propene.
- Identify the compound that does not fit the alkane formula: Based on the calculations, \(C_3H_6\) is not an alkane.
Correct Option: D) \(C_3H_6\) is not an alkane because it follows the general formula \(C_nH_{2n}\), which is characteristic of alkenes (compounds with at least one carbon-carbon double bond), not alkanes.
Incorrect Options
- A) \(C_2H_6\) — This compound fits the general formula for alkanes \(C_nH_{2n+2}\) where \(n=2\) (ethane). Thus, it is an alkane.
- B) \(C_4H_{10}\) — This compound fits the general formula for alkanes \(C_nH_{2n+2}\) where \(n=4\) (butane). Thus, it is an alkane.
- C
6.
The IUPAC name of (CH3)2CHCH2CH3 is:
Approach
To determine the IUPAC name of an organic compound, we follow a systematic set of rules. These rules involve identifying the longest continuous carbon chain, numbering the chain to give substituents the lowest possible numbers, and then naming the substituents alphabetically.
Step-by-step
- Identify the longest continuous carbon chain: The given compound is \((CH_3)_2CHCH_2CH_3\). Let's expand its structure:
\[ \begin{array}{c} \quad CH_3 \ \quad | \ CH_3 - CH - CH_2 - CH_3 \end{array} \]Counting the carbons in the longest continuous chain:- If we count from left to right: \(CH_3 - CH(CH_3) - CH_2 - CH_3\). The main chain is \(CH - CH_2 - CH_3\), which is 3 carbons, with a \(CH_3\) branch. This is incorrect.
- Let's consider the chain starting from the top \(CH_3\), through the \(CH\), then \(CH_2\), and finally \(CH_3\). This gives a chain of 4 carbons.
- Another way: starting from the leftmost \(CH_3\), through the \(CH\), then \(CH_2\), and finally \(CH_3\). This also gives a chain of 4 carbons.
The longest continuous carbon chain has 4 carbon atoms. This means the parent alkane is butane. - Number the carbon chain: We need to number the chain such that the substituent gets the lowest possible number. If we number from left to right (starting from the \(CH_3\) attached to the \(CH\)):
\[ \begin{array}{c} \quad CH_3 \ \quad | \ \overset{1}{C}H_3 - \overset{2}{C}H - \overset{3}{C}H_2 - \overset{4}{C}H_3 \end{array} \]In this numbering, the methyl group is at position 2.If we number from right to left:
\[ \begin{array}{c} \quad CH_3 \ \quad | \ \overset{4}{C}H_3 - \overset{3}{C}H - \overset{2}{C}H_2 - \overset{1}{C}H_3 \end{array} \]In this numbering, the methyl group is at position 3.According to IUPAC rules, we choose the numbering that gives the substituent the lowest possible number. So, the methyl group is at position 2. - Identify and name the substituent: There is one methyl group (\(CH_3\)) attached to the second carbon atom.
- Construct the full name: Combine the position of the substituent, the name of the substituent, and the name of the parent alkane. This gives 2-Methylbutane.
Correct Option:
D) 2-Methylbutane — This is the correct IUPAC name as determined by identifying the longest carbon chain (butane) and the position of the methyl substituent (at carbon 2).
Incorrect Options
- A) 3-Methylbutane — This is incorrect because the numbering rule states that the substituent should have the lowest possible number. Numbering from the other end would place the methyl group at position 3, which
