Question

give one possibility for the mystery reactant r in this organic reaction:

r + h₂ → (ni catalyst) product skeletal structure

specifically, in the drawing area below draw the skeletal (\line\) structure of what r might be. there may be more than one correct answer.
note: keep in mind that the equation above states r and h₂ are present in a 1 : 1 mole ratio.

drawing area with click and drag to start drawing a structure. and a toolbar

no reaction checkbox

Explanation:

Step1: Identify the product structure

The product is 2 - methylbutane (skeletal structure: a central carbon with a methyl group, and four carbons in total: the central carbon (C2) is bonded to C1, C3, a methyl (CH₃), and the product of hydrogenation. The product's formula is \( C_5H_{12} \) (since it's a saturated alkane, \( C_nH_{2n + 2} \), \( n = 5 \), so \( 2\times5+ 2=12 \) hydrogens).

Step2: Determine the reactant type

The reaction is hydrogenation (addition of \( H_2 \) over \( Ni \) catalyst), so the reactant \( R \) should be an unsaturated compound (alkene or alkyne) that reacts with 1 mole of \( H_2 \) (1:1 mole ratio). Since it's 1:1 with \( H_2 \), it's an alkene (because an alkyne would react with 2 moles of \( H_2 \) for full saturation).

Step3: Find the alkene isomer

The product is 2 - methylbutane. To find the alkene, we can introduce a double bond into the carbon chain of 2 - methylbutane. The possible alkenes that can be hydrogenated to 2 - methylbutane with 1 mole of \( H_2 \) are:
- 2 - methyl - 1 - butene: Skeletal structure: \( \ce{CH2=CH(CH3)CH2CH3} \) (but in skeletal form: a double bond between C1 and C2, with C2 bonded to a methyl group)
- 2 - methyl - 2 - butene: Skeletal structure: \( \ce{(CH3)2C=CHCH3} \) (double bond between C2 and C3, C2 has two methyl groups? Wait, no, 2 - methylbutane has the structure: C1 - C2 (with methyl) - C3 - C4? Wait, correct carbon numbering for 2 - methylbutane: C1: \( \ce{CH3} \), C2: \( \ce{CH(CH3)} \), C3: \( \ce{CH2} \), C4: \( \ce{CH3} \)? No, 2 - methylbutane is \( \ce{CH3 - CH(CH3) - CH2 - CH3} \) (carbon chain: C1 - C2 (with methyl) - C3 - C4? Wait, no, the correct structure is 5 carbons? Wait, no, 2 - methylbutane is \( C_5H_{12} \), so the carbon chain is: C1: \( \ce{CH3} \), C2: \( \ce{CH(CH3)} \), C3: \( \ce{CH2} \), C4: \( \ce{CH3} \)? No, that's 4 carbons? Wait, no, 2 - methylbutane is also called isopentane, with structure: \( \ce{CH3 - CH(CH3) - CH2 - CH3} \) is wrong, actually, it's \( \ce{CH3 - CH(CH3) - CH2 - CH3} \) has 5 carbons? Wait, no, \( \ce{CH3 - CH(CH3) - CH2 - CH3} \) is 4 carbons? No, no: C1: \( \ce{CH3} \), C2: \( \ce{CH(CH3)} \), C3: \( \ce{CH2} \), C4: \( \ce{CH3} \) – that's 4 carbons? No, that's a mistake. 2 - methylbutane is \( \ce{CH3 - CH(CH3) - CH2 - CH3} \) is actually 5 carbons? Wait, no, the formula is \( C_5H_{12} \), so the correct structure is: C1: \( \ce{CH3} \), C2: \( \ce{CH(CH3)} \), C3: \( \ce{CH2} \), C4: \( \ce{CH3} \) – no, that's 4 carbons with a methyl on C2, total carbons: 5 (C1, C2, C3, C4, and the methyl on C2 is C5? Wait, no, the methyl group on C2 is \( \ce{CH3} \), so C2 is bonded to C1, C3, and two methyl groups? No, 2 - methylbutane is \( \ce{(CH3)2CH - CH2 - CH3} \), so carbon atoms: C1 (of the first methyl), C2 (the central carbon with two methyls? No, \( \ce{(CH3)2CH - CH2 - CH3} \) is: C1: \( \ce{CH3} \), C2: \( \ce{CH(CH3)} \), C3: \( \ce{CH2} \), C4: \( \ce{CH3} \) – no, that's 4 carbons? No, \( \ce{(CH3)2CH - CH2 - CH3} \) has 5 carbons: the two methyls on C2 are two carbons? No, each methyl is one carbon. So C2 is bonded to two methyls (C1 and C5), C3, and C4? Wait, I think I messed up the numbering. Let's do it properly:

The product is 2 - methylbutane, which has the IUPAC name 2 - methylbutane, so the parent chain is butane (4 carbons: C1 - C2 - C3 - C4), and a methyl group on C2. So the structure is:

C1: \( \ce{CH3} \)

C2: \( \ce{CH(CH3)} \) (bonded to C1, C3, and \( \ce{CH3} \))

C3: \( \ce{CH2} \)

C4: \( \ce{CH3} \)

Now, to form this by hydrogenating an alkene, we can introduce a double bond between any two adjacent carbons where adding \( H_2 \) (i.e., adding two hydrogens, one to each carbon of the double bond) will result in 2 - methylbutane.

Possible alkenes:

1. 2 - methyl - 1 - butene: Double bond between C1 and C2. Structure: \( \ce{CH2=CH(CH3)CH2CH3} \) (skeletal: \( \ce{=CH - CH(CH3) - CH2 - CH3} \) – wait, skeletal structure: the double bond is at the end (C1 - C2), with C2 having a methyl group.

2. 2 - methyl - 2 - butene: Double bond between C2 and C3. Structure: \( \ce{(CH3)2C=CHCH3} \) (skeletal: \( \ce{(CH3)2C=CH - CH3} \))

3. 3 - methyl - 1 - butene: Double bond between C1 and C2, but the methyl is on C3? Wait, no, 3 - methyl - 1 - butene would be \( \ce{CH2=CH - CH(CH3)2} \), but hydrogenating that would give \( \ce{CH3 - CH2 - CH(CH3)2} \), which is 2 - methylbutane? Wait, no, \( \ce{CH3 - CH2 - CH(CH3)2} \) is 2 - methylbutane? Wait, no, \( \ce{CH3 - CH2 - CH(CH3)2} \) is 2 - methylbutane? Wait, no, the parent chain is butane, so 2 - methylbutane is \( \ce{(CH3)2CH - CH2 - CH3} \), and \( \ce{CH3 - CH2 - CH(CH3)2} \) is also 2 - methylbutane (same compound, just different numbering? No, IUPAC numbering: the parent chain is the longest chain, so for \( \ce{CH3 - CH2 - CH(CH3)2} \), the longest chain is 4 carbons (C1 - C2 - C3 - C4), with a methyl on C3? Wait, no, I'm getting confused. Let's use the formula: the product is \( C_5H_{12} \), so the alkene must be \( C_5H_{10} \) (since adding \( H_2 \) (2 hydrogens) to an alkene \( C_nH_{2n} \) gives \( C_nH_{2n + 2} \); here \( n = 5 \), so alkene is \( C_5H_{10} \)).

So possible alkenes with formula \( C_5H_{10} \) that can be hydrogenated to 2 - methylbutane (\( C_5H_{12} \)) are the isomers of pentene where the double bond is in a position that, when hydrogenated, gives the 2 - methylbutane structure.

The correct alkenes are:

- 2 - methyl - 1 - butene: \( \ce{CH2=CH - CH(CH3)2} \) (wait, no, \( \ce{CH2=CH - CH(CH3)2} \) is 3 - methyl - 1 - butene? Wait, I think I need to draw the skeletal structure.

Skeletal structure of 2 - methylbutane:

```
CH3
|
CH3 - C - CH2 - CH3
|
H (but in skeletal, we don't draw H, just lines)
```

So the carbon chain is:

```
C
|
C - C - C - C
|
C
```

Wait, no, skeletal structure for 2 - methylbutane (isopentane) is:

```
C
|
C - C - C - C
|
C
```

Wait, no, the correct skeletal structure is:

```
CH3
|
CH3 - CH - CH2 - CH3
```

In skeletal (line) structure, this is:

```
C
|
C - C - C - C
|
C
```

Now, to introduce a double bond:

Option 1: Double bond between the first and second carbon (from the left):

```
C
|
= C - C - C - C
|
C
```

This is 2 - methyl - 1 - butene (the double bond is at the end, between C1 and C2, with C2 having a methyl group).

Option 2: Double bond between the second and third carbon:

```
C
|
C = C - C - C
|
C
```

This is 2 - methyl - 2 - butene (double bond between C2 and C3, with C2 having two methyl groups? Wait, no, in the skeletal structure, the second carbon (from the left) is bonded to a methyl (top) and a methyl (bottom)? No, in the product, the second carbon is bonded to one methyl (top) and the first and third carbons. So in the alkene, if we have a double bond between C2 and C3, the structure is:

```
CH3
|
CH3 - C = CH - CH3
```

Which is 2 - methyl - 2 - butene.

Either of these alkenes can react with 1 mole of \( H_2 \) over \( Ni \) to form 2 - methylbutane.

Let's choose 2 - methyl - 2 - butene as an example. Its skeletal structure is:

```
C
|
C = C - C - C
|
C
```

Or in more standard skeletal drawing:

```
CH3
|
CH3 - C = CH - CH3
```

In line (skeletal) structure, we can draw it as:

```
Me
|
Me - C = C - Me
|
H? No, skeletal structure omits H on carbons. Wait, correct skeletal structure for 2 - methyl - 2 - butene is:

```
C
|
C = C - C - C
|
C
```

Where the first C (left) is bonded to a methyl (top), the second C is double - bonded to the third C, and the fourth C is bonded to a methyl (bottom). Wait, maybe a better way: the product is 2 - methylbutane, so the reactant \( R \) is an alkene with the same carbon skeleton, just with a double bond. So one possible \( R \) is 2 - methyl - 2 - butene, whose skeletal structure is:

```
CH3
|
CH3 - C = CH - CH3
```

In line structure (skeletal), we can represent it as:

```
Me
|
Me - C = C - Me
```

But in the drawing area, we need to draw the skeletal structure. So the steps are:

1. Recognize that the reaction is hydrogenation (alkene + \( H_2 \) → alkane).

2. The product is 2 - methylbutane (\( C_5H_{12} \)), so the alkene must be \( C_5H_{10} \) (since \( H_2 \) adds 2 H atoms).

3. Find an alkene isomer of \( C_5H_{10} \) that has the same carbon skeleton as 2 - methylbutane (i.e., a 4 - carbon chain with a methyl on the second carbon, or a 5 - carbon chain? Wait, no, 2 - methylbutane has 5 carbons: the parent chain is 4 carbons, plus one methyl, total 5. So the alkene must also have 5 carbons.

4. The possible alkenes are 2 - methyl - 1 - butene, 2 - methyl - 2 - butene, and 3 - methyl - 1 - butene.

5. Choose one, e.g., 2 - methyl - 2 - butene, and draw its skeletal structure.

Answer:

One possible reactant \( R \) is 2 - methyl - 2 - butene, whose skeletal structure is:

    CH3
    |
CH3 - C = CH - CH3

(In skeletal line structure, this is represented as:

    C
    |
C = C - C - C
    |
    C

where the first carbon (left) has a methyl (top), the second carbon is double - bonded to the third carbon, and the fourth carbon has a methyl (bottom).)