Parent chains

The parent chain must contain the highest-priority functional group, and among such chains the longest wins, traced through branches as well as the written line.

Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.

1. What you will learn

You will find the parent chain of a structure, order functional groups by priority, choose the group that names a molecule, and trace the longest chain through a branch.

2. What you already have

You can read a condensed formula and find its functional groups. A name starts by choosing which chain everything else hangs from. Structures in this course are written as condensed formulas, read left to right: CH3CH2OH is a CH3 joined to a CH2 joined to an OH. Brackets hang a group off the carbon before them, so CH3CH(CH3)CH3 has a CH3 branch on its middle carbon.

3. Words for this lesson

The parent chain is the chain a name is built on. A substituent is a group hanging off it. The suffix names the highest-priority functional group (-ol, -one, -oic acid); prefixes name everything else (methyl-, hydroxy-).

4. Two rules, in order

Rule 1: the parent chain contains the highest-priority functional group. Priority, highest first:

carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene, alkyne > alkane.

Rule 2: among chains that satisfy rule 1, the longest wins.

For an alkane there is no functional group, so rule 2 alone decides. But the longest chain is not always the one written in a line. CH3CH(CH2CH2CH3)CH2CH3 is written with four carbons in a row, yet the path that runs out along the bracketed propyl group and back along the main line has six: it is a hexane.

Another way: steps

To choose a parent:

Find every functional group; pick the highest priority.
Consider only chains containing its carbon.
Trace every end-to-end path among them, through branches too.
The longest is the parent.

5. Why the group comes first

The name's suffix announces the molecule's main functional group, so that group must sit on the parent. In CH3CH2CH2CH(CH2OH)CH2CH3 a six-carbon chain exists, but it misses the –OH carbon; the parent is the five-carbon chain through it, and the name is 2-ethylpentan-1-ol. Naming it as a hexane would hide the fact that it is an alcohol. When the group is a ring or several groups tie, further rules apply, but these two settle nearly everything you will meet.

6. Where this goes wrong

The longest carbon chain always sets the name regardless of functional group. The group comes first.

The chain is the written line. Trace through the branches.

Every group can be a prefix. The highest-priority group is the suffix.

An alcohol outranks a ketone. The carbonyl ranks higher.

7. 2,4-dimethylpentane, CH3CH(CH3)CH2CH(CH3)CH3

No functional group, so only length matters.
Rule 2 alone.
The longest path runs end to end through five carbons; the two bracketed CH3 groups are branches.
Trace every path.
Parent: pentane, with methyl groups on carbons 2 and 4.
Seven carbons in all.

8. Your turn: HOCH2CH2CH2COOH

Which group has the higher priority?
The carboxylic acid.
So the suffix is -oic acid and the –OH is a prefix. How long is the parent?
Your turn: work this step out. Its working is at the end of the packet.
Four carbons, including the acid carbon: 4-hydroxybutanoic acid.

9. Guided practice

How many carbons are in the parent chain of CH3CH(CH3)CH2CH3?

Answer:

10. Guided practice

Put these functional groups in order of naming priority, highest first.

Number the steps in order (write the number in the box):

carboxylic acid, –COOH
aldehyde, –CHO
ketone, C=O within a chain
alcohol, –OH
amine, –NH2

11. Practice

Each molecule has two functional groups. Match it to the group that becomes the suffix of its name.

	the carboxylic acid	the ketone	the alcohol	the aldehyde
HOCH2CH2COOH
CH3COCH2CH2OH
H2NCH2CH2OH
OHCCH2COCH3

12. Practice

In CH3CH2CH2CH(CH2OH)CH2CH3, the longest carbon chain has six carbons, but the –OH hangs off a CH2 in the branch. Which chain is the parent?

The longest chain that contains the carbon bearing the –OH, five carbons
The six-carbon chain, because the longest chain always wins
Any chain, since the –OH can be named as a substituent
The written line, left to right, whatever its length

13. Somewhere new

A formula is written CH3CH(CH2CH2CH3)CH2CH3. How many carbons are in its parent chain?

Answer:

14. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

15. Test question

2-methylpentane is CH3CH(CH3)CH2CH2CH3. How many of its carbons lie outside the parent chain, in branches?

The number of carbons outside the parent chain is _a.

16. What you can do now

You can choose the chain a name is built on. Tell someone why the six-carbon chain is not always the parent. Next: naming a branched alkane in full.

Working for the steps left to you

8. Your turn: HOCH2CH2CH2COOH, step 3