Chemistry · General chemistry II · Worked example

Tell whether two drawings are one compound

Are ${CH}_{3} CH ({CH}_{3}) {CH}_{2} {CH}_{3}$ and ${CH}_{3} {CH}_{2} CH ({CH}_{3}) {CH}_{3}$ the same compound? Is ${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{3}$ the same as either?

Compare the formulas

Each has $5$ carbons. The first has $3 + 1 + 3 + 2 + 3 = 12$ hydrogens, and so do the others, so all three are $C_{5} H_{12}$ . Matching formulas cannot decide, so the connections must.

3 + 1 + 3 + 2 + 3 = 12

Trace the first two

The first is a four-carbon chain with a ${CH}_{3}$ branch on its second carbon. Read the second from right to left and it is ${CH}_{3} CH ({CH}_{3}) {CH}_{2} {CH}_{3}$ , the first one exactly. Reading a chain backward is allowed because a bond joins two atoms whichever one you write first.

Trace the third

${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{3}$ is a five-carbon chain with no branch. No reading of it has a carbon bonded to three others, so its connections differ from the first two.

Settle it with names

Both of the first two are 2-methylbutane, and the third is pentane. In SMILES they are CC(C)CC, CCC(C)C and CCCCC, and Chalk names them the same way.

Three $C_{5} H_{12}$ drawings
Condensed	SMILES	Name
${CH}_{3} CH ({CH}_{3}) {CH}_{2} {CH}_{3}$	CC(C)CC	2-methylbutane
${CH}_{3} {CH}_{2} CH ({CH}_{3}) {CH}_{3}$	CCC(C)C	2-methylbutane
${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{3}$	CCCCC	pentane

Result

The first two are the same compound, 2-methylbutane, written from opposite ends. ${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{3}$ is pentane, a constitutional isomer of it.

Your turn

1. One like the example

Are ${CH}_{3} {CH}_{2} CH ({CH}_{3}) {CH}_{2} {CH}_{3}$ and ${({CH}_{3} {CH}_{2})}_{2} {CHCH}_{3}$ the same compound?

Show the answer and explanation

Yes. Both are 3-methylpentane.

In each, one carbon is bonded to two ethyl groups and a methyl group, which makes a five-carbon chain with a methyl on carbon 3. Both are $C_{6} H_{14}$ , with $3 + 2 + 1 + 3 + 2 + 3$ $= 14$ hydrogens, and $2 (6) + 2 = 14$ agrees.

\begin{array}{l} 3 + 2 + 1 + 3 + 2 + 3 = 14 \\ 2 (6) + 2 = 14 \end{array}

2. A variation

Are ${CH}_{3} CH ({CH}_{3}) {CH}_{2} {CH}_{2} {CH}_{3}$ and ${CH}_{3} {CH}_{2} CH ({CH}_{3}) {CH}_{2} {CH}_{3}$ the same compound?

Show the answer and explanation

No. They are 2-methylpentane and 3-methylpentane, constitutional isomers.

Both are $C_{6} H_{14}$ with a five-carbon chain and one methyl branch. In the first the branch is on carbon 2 from the nearer end, and in the second it is on carbon 3 from either end. No reading of one gives the other.

3. Somewhere new

C/C=C/C and C/C=C\C have the same formula and the same connections. Are they the same compound, and which representations show the difference?

Show the answer and explanation

No. They are trans-but-2-ene and cis-but-2-ene, stereoisomers. A skeletal drawing, the marked SMILES and the full name show it. The formula, ${CH}_{3} CH = {CHCH}_{3}$ and plain CC=CC do not.

A double bond does not rotate, so the two methyl groups stay on opposite sides in one and the same side in the other. The current IUPAC names say it with E and Z, (2E)-but-2-ene and (2Z)-but-2-ene. Chalk’s Compare calls the pair “Cis/trans isomers, as drawn”.

Keep exploring

Open the Skeletal tool, import CC(C)CC and CCC(C)C as two drawings and choose Analyze → Compare: Chalk reports “Same compound”. Compare CC(C)CC with CCCCC and it reports “Constitutional isomers”. Each practice pair can be checked the same way, with your own SMILES for each formula imported as two drawings.

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Sources and scope

Authored study material. Tool results depend on the stated inputs and model assumptions.

After reading

Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 4 key terms Import CC(C)CC into the Skeletal tool Import CCC(C)C beside it Import pentane, CCCCC, beside them Open worked example on a board Organic nomenclature in Chemistry Reference

Your existing work stays on this device. Examples open as editable copies.

Compare the formulas

Trace the first two

Trace the third

Settle it with names

Result

Your turn

1. One like the example

Yes. Both are 3-methylpentane.

2. A variation

No. They are 2-methylpentane and 3-methylpentane, constitutional isomers.

3. Somewhere new

No. They are trans-but-2-ene and cis-but-2-ene, stereoisomers. A skeletal drawing, the marked SMILES and the full name show it. The formula, CH3CH=CHCH3CH3​CH=CHCH3​ and plain CC=CC do not.

Keep exploring

Study it, then try it

No. They are trans-but-2-ene and cis-but-2-ene, stereoisomers. A skeletal drawing, the marked SMILES and the full name show it. The formula, ${CH}_{3} CH = {CHCH}_{3}$ and plain CC=CC do not.