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An alkane's name is its branches, numbered from the end giving the lower locants and listed alphabetically, in front of the parent; a name also works backwards as instructions for a structure.
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.
You will name branched alkanes from their formulas, choose the lowest locants, count an alkane's hydrogens, name straight chains from their length, and match names to structures.
You can choose a parent chain, tracing through branches. Naming finishes the job: numbers for the branches, and a fixed order to write them in.
A stem says how many carbons a chain has: meth 1, eth 2, prop 3, but 4, pent 5, hex 6, hept 7, oct 8. An alkyl group is a branch named from its stem with -yl: methyl, ethyl. A locant is the number of the carbon a branch sits on. Alkanes have the formula CnH2n+2.
Name CH3CH2CH(CH3)CH2CH3:
More than one of the same branch takes di-, tri-, tetra-, and every locant is listed: CH3C(CH3)2CH2CH3 is 2,2-dimethylbutane, with a comma between numbers and a hyphen between a number and a word. Different branches go in alphabetical order, ignoring di- and tri-: 3-ethyl-2-methylhexane, not 2-methyl-3-ethylhexane.
Another way: table
The first eight stems.
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Stem | meth | eth | prop | but | pent | hex | hept | oct |
When numbering, compare the locant sets from each end at the first point of difference and keep the lower. CH3CH2CH2CH(CH3)CH3 numbered from the left puts the methyl on carbon 4; from the right, on carbon 2: it is 2-methylpentane.
A name is also a set of instructions. 2,3-dimethylbutane says: draw four carbons in a row, put a CH3 on the second and third, fill with hydrogens — CH3CH(CH3)CH(CH3)CH3. If your drawing's longest chain is not the parent the name claims, the name was wrong: '1-methylbutane' is really pentane.
Substituent numbers can be chosen from either end without comparison. Compare and keep the lower set.
Branches are listed in number order. They are listed alphabetically.
Di- counts in the alphabet. It is ignored.
A branch can sit on carbon 1. Then it is part of the chain.
Parent: five carbons, a pentane.
The longest chain.
Methyls on carbons 2 and 4, whichever end you start from.
Lowest locants.
Assembled, the name is 2,4-dimethylpentane.
Two methyls: dimethyl, both numbers listed.
How long is the parent chain?
Five carbons.
What branch sits on which carbon?
An ethyl on carbon 3: 3-ethylpentane.
Give the IUPAC name of CH3CH(CH3)CH3.
The name is n.
pentane has $5$ carbon atoms. How many hydrogen atoms does it have?
Answer:
CH3CH2CH2CH(CH3)CH3 has a five-carbon parent chain with one methyl branch. Which name is correct?
Put the steps for naming a branched alkane in order.
Number the steps in order (write the number in the box):
A fuel component is an unbranched alkane with $1$ carbon atoms in a row. What is its name?
It is n.
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Match each name to its structure.
| CH3C(CH3)2CH2CH3 | CH3CH(CH3)CH(CH3)CH3 | CH3CH2CH(CH3)CH2CH3 | CH3CH(CH3)CH2CH2CH3 | |
|---|---|---|---|---|
| 2,2-dimethylbutane | ||||
| 2,3-dimethylbutane | ||||
| 3-methylpentane | ||||
| 2-methylpentane |
You can translate an alkane between name and structure in both directions. Tell someone why 4-methylpentane is not a name. Next: the suffixes and prefixes that name functional groups.
8. Your turn: CH3CH2CH(CH2CH3)CH2CH3, step 3