SME
Organic Chemistry I
Radical halogenation
Use this for “Propane reacts with Cl₂ in light at 25 °C, and each 2° H is 3.8 times as reactive as each 1° H. What percent of each monochloride forms?”
Chart
This is an example. Type your own values to start a new problem.
n₁
Primary H
example
n₂
Secondary H
example
n₃
Tertiary H
example
r₁
Primary reactivity
example
r₂
Secondary reactivity
example
r₃
Tertiary reactivity
example
Σ
Total of H × reactivity
worked out (not typed)
P₁
Product from 1° H
calculated · %
P₂
Product from 2° H
calculated · %
P₃
Product from 3° H
calculated · %
Formulas
Σ
=
n₁
×
r₁
+
n₂
×
r₂
+
n₃
×
r₃
13.6 = 6 × 1 + 2 × 3.8 + 0 × 5
P₁
=
100
×
n₁
×
r₁
Σ
44.1
=
100
×
6
×
1
13.6
P₂
=
100
×
n₂
×
r₂
Σ
55.9
=
100
×
2
×
3.8
13.6
P₃
=
100
×
n₃
×
r₃
Σ
0
=
100
×
0
×
5
13.6
Assumptions
•
Each H reacts at its class’s reactivity, so each product’s share is its H count × that reactivity, out of the total.
•
The reactivities are typed: primary 1, secondary 3.8 and tertiary 5 for chlorination at 25 °C; for Br₂ use 1, 82 and 1600.
•
One H is replaced in each molecule: the monohalides only, none twice substituted.
Step-by-step
WE KNOW
n₁ = 6, n₂ = 2, n₃ = 0, r₁ = 1, r₂ = 3.8, r₃ = 5
FIND
total of H × reactivity (Σ), product from 1° H (P₁), product from 2° H (P₂), product from 3° H (P₃)
Step 1 · Find total of H × reactivity (Σ)
Use Σ = n₁ × r₁ + n₂ × r₂ + n₃ × r₃
Multiply each class’s H count by its reactivity, then add.
Σ
=
6
×
1
+
2
×
3.8
+
0
×
5
Σ
=
6
+
7.6
Σ
=
13.6
Step 2 · Find product from 1° H (P₁)
Use P₁ = 100 × n₁ × r₁ ÷ Σ
The primary H’s part of the total, as a percent.
P₁
=
100
×
6
×
1
13.6
P₁
=
600
13.6
P₁
=
44.1%
Step 3 · Find product from 2° H (P₂)
Use P₂ = 100 × n₂ × r₂ ÷ Σ
The secondary H’s part of the total, as a percent.
P₂
=
100
×
2
×
3.8
13.6
P₂
=
200
×
3.8
13.6
P₂
=
760
13.6
P₂
=
55.9%
Step 4 · Find product from 3° H (P₃)
Use P₃ = 100 × n₃ × r₃ ÷ Σ
The tertiary H’s part of the total, as a percent.
P₃
=
100
×
0
×
5
13.6
P₃
=
0
×
5
13.6
P₃
=
0
13.6
P₃
=
0%
→
The
halide
from
the
secondary
H
is
the
major
product,
55.9%
of
the
mix.
Check
Put the numbers back in. Both sides should match.
13.6 = 6 × 1 + 2 × 3.8 + 0 × 5 ✓
44.1
=
100
×
6
×
1
13.6
✓
55.9
=
100
×
2
×
3.8
13.6
✓
0
=
100
×
0
×
5
13.6
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
More problem types
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