Consider the following reactions
`A underset((ii)H_(3)O^(+))overset((i)CH_(3)MgBr)to B underset(573K)overset(Cu)to 2`-methyl-2-butene
the mass percentage of carbon in A is ________.

Options

A. 33.33
B. 6.67
C. 16.66
D. 66.67

Answer

To solve the problem, we need to determine the mass percentage of carbon in compound A, which is formed in the reaction sequence leading to the production of 2-methyl-2-butene.

### Step-by-step Solution:

1. **Identify Compound A**:
Based on the reactions provided, we can deduce that compound A is likely to be a ketone that can lead to the formation of 2-methyl-2-butene. A suitable structure for A is 3-pentanone (CH3C(=O)CH2CH3), which has the formula C5H10O.

2. **Count the Atoms in Compound A**:
- Carbon (C): 5
- Hydrogen (H): 10
- Oxygen (O): 1

3. **Calculate the Molar Mass of Compound A**:
The molar mass can be calculated as follows:
- Molar mass of Carbon (C) = 12 g/mol
- Molar mass of Hydrogen (H) = 1 g/mol
- Molar mass of Oxygen (O) = 16 g/mol

Therefore, the molar mass of A (C5H10O) is:
\[ \text{Molar mass of A} = (5 \times 12) + (10 \times 1) + (1 \times 16) = 60 + 10 + 16 = 86 \text{ g/mol} \]

4. **Calculate the Total Mass of Carbon in Compound A**:
The total mass of carbon in compound A is:
\[ \text{Total mass of Carbon} = 5 \times 12 = 60 \text{ g} \]

5. **Calculate the Mass Percentage of Carbon in Compound A**:
The mass percentage of carbon is calculated using the formula:
\[ \text{Mass percentage of Carbon} = \left( \frac{\text{Mass of Carbon}}{\text{Molar mass of A}} \right) \times 100 \]
Substituting the values we calculated:
\[ \text{Mass percentage of Carbon} = \left( \frac{60}{86} \right) \times 100 \approx 69.77\% \]

6. **Final Answer**:
The mass percentage of carbon in A is approximately 69.77%.