SME
Grade 10 Science · Problem type
Isomers: a branched alkane
Use this for “Draw an isomer of pentane” or “How many hydrogens does 2-methylbutane have?”
Diagram
2-methylbutane, C₅H₁₂: a chain of 4 carbons with 1 methyl group.
Carbons = 4 + 1 = 5; hydrogens = 2 × 5 + 2 = 12.
Every carbon makes 4 bonds and every hydrogen 1.
This is an example. Choose your own values to start a new problem.
C
H
n
Carbons in the main chain
example
Formulas
c
=
n
+
1
5 = 4 + 1
h
=
2
×
c
+
2
12 = 2 × 5 + 2
Assumptions
•
The main chain is the longest chain of carbons; a methyl group, CH₃, hangs off its second carbon.
•
Isomers have the same formula but different structures: 2-methylbutane and pentane are both C₅H₁₂.
Step-by-step
WE KNOW
n = 4
FIND
carbons in all (c), hydrogen atoms (h)
Step 1 · Find carbons in all (c)
Use c = n + 1
The methyl group adds one carbon to the main chain’s carbons.
c
=
4
+
1
c
=
5
Step 2 · Find hydrogen atoms (h)
Use h = 2 × c + 2
Any alkane, branched or not, is CₙH₂ₙ₊₂.
h
=
2
×
5
+
2
2-methylbutane, C₅H₁₂: the same formula as pentane
h
=
12
Check
Put the numbers back in. Both sides should match.
5 = 4 + 1 ✓
12 = 2 × 5 + 2 ✓
Topic check
See what you know: 5 questions on this topic
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Related lessons
Organic chemistry: hydrocarbons, functional groups and polymers
Main lesson
Alkenes: one double bond
Use this for an alkene’s formula and name: a chain with one carbon–carbon double bond, CₙH₂ₙ.
Alkynes: one triple bond
Use this for an alkyne’s formula and name: a chain with one carbon–carbon triple bond, CₙH₂ₙ₋₂.
Functional groups
Use this for “Which functional group does ethyl acetate have?”
Addition or condensation polymer?
Use this for “Is nylon an addition polymer or a condensation polymer?”