Apply ortho and para directing effects
Predict the main ring positions directed by –NH₂, –OH and alkyl substituents during electrophilic substitution.
Section 30 · Hydrocarbons
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 30. Follow the listed reagents, catalysts, temperatures and reaction scope for benzene and methylbenzene. Directing effects are limited to –NH₂, –OH, –R, –NO₂, –COOH and –COR. Written practice is self-checked
Before you begin
- Use the Topic 29 model of benzene as a planar aromatic ring with a delocalised π system, and recognise electrophiles, nucleophiles and organic functional groups.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which positions are ortho to a substituent at carbon 1?
What you will learn
- Identify ortho, meta and para positions relative to a substituent
- Predict ortho/para direction from –NH₂, –OH and –R
- Relate these directing effects qualitatively to electron donation
Locate positions relative to the first substituent
For a substituent at carbon 1, ortho positions are 2 and 6, meta positions are 3 and 5, and the para position is 4.
The syllabus lists –NH₂, –OH and –R as ortho/para directors. A second electrophile is therefore directed mainly toward carbons 2, 4 or 6 relative to that group; the exact product distribution can depend on conditions.
Use qualitative electron donation
The lone pairs on –NH₂ and –OH can donate electron density into the aromatic system; alkyl groups also donate weakly. This makes the ortho and para positions comparatively favourable for electrophilic attack in the syllabus model.
Use this as a directing rule, not a full quantitative prediction of rates or product ratios. Do not include substituents outside the syllabus list unless supplied in a question.
Pause and explain
What directing effect does –OH have in the listed syllabus set?
Worked example
Where does a methyl group direct nitration of methylbenzene?
Show the worked solution
- Treat the existing methyl group as position 1.
- –R is an ortho/para director.
- The nitro group is directed mainly to positions 2 and 4 (also position 6, equivalent to 2).
Answer Methyl directs nitration mainly to the ortho and para positions, giving 2- and 4-nitromethylbenzene as the distinct ring-position products.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For phenol numbered with –OH at position 1, name the positions to which a second substituent is directed.
Give me a hint
Use the ortho/para pattern for –OH.
Compare my reasoning
- Ortho positions are 2 and 6.
- Para position is 4.
- The listed prediction is 2, 4 and 6.
The –OH group directs mainly to positions 2, 4 and 6.
Look for these in your work
- Ortho positions correct
- Para position correct
- Full set stated
A methyl group is at position 1 of a benzene ring. Is position 3 ortho, meta or para?
Give me a hint
Count around the ring from position 1.
Compare my reasoning
- Positions 2 and 6 are one carbon away.
- Position 3 is two steps from position 1.
- That makes position 3 meta.
Position 3 is meta to the methyl group; –R directs mainly ortho and para, not meta.
Look for these in your work
- Numbering traced
- Relationship classified
- Directing rule contrasted
A question asks for the major directing positions after nitration of an aromatic compound with –NH₂ at position 1. What positions should you predict and what should you avoid claiming?
Give me a hint
Apply the listed director while keeping selectivity qualitative.
Compare my reasoning
- –NH₂ is an ortho/para director.
- Predict positions 2, 4 and 6.
- Do not claim an exact ratio without supplied evidence.
Predict mainly positions 2, 4 and 6; the directing rule does not by itself establish exact product proportions.
Look for these in your work
- Director applied
- Positions named
- Quantitative limit kept
Common mistakes
- Calling –R a meta director
- Mixing up meta position 3 with para position 4
- Counting substituents from an arbitrary atom
- Claiming the directing rule alone fixes a single product ratio
What can you explain now?
Which positions are ortho to a substituent at carbon 1?
Compare with the explanation
2 and 6
The two adjacent ring positions are ortho.
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Make it stick
How do –NH₂, –OH and –R influence where a new electrophile enters the ring?
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