Isomerism: structural and stereoisomers (HL)
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Question
What are isomers?
Answer
Different compounds with the **same molecular formula** but a different arrangement of atoms.
Question
Structural vs stereoisomers?
Answer
**Structural** isomers differ in **connectivity** (which atom bonds to which); **stereoisomers** have the same connectivity but a different **arrangement in space**.
Question
Name the three types of structural isomerism.
Answer
**Chain** (different carbon skeleton), **position** (group in a different place), and **functional-group** (a different functional group/family).
Question
Give an example of functional-group isomerism.
Answer
Ethanol (CH_{3}CH_{2}OH, an alcohol) and methoxymethane (CH_{3}OCH_{3}, an ether) — both C_{2}H_{6}O.
Question
What two conditions are needed for cis/trans isomerism?
Answer
A **C=C double bond** (restricted rotation) **and** two **different** groups on **each** doubly-bonded carbon.
Question
Difference between cis and trans?
Answer
**cis** = the two like groups on the **same** side of the C=C; **trans** = on **opposite** sides.
Question
Why does but-2-ene show cis/trans but but-1-ene does not?
Answer
But-2-ene has a CH_{3} and an H on each C=C carbon (two different groups each); but-1-ene has two identical H atoms on one carbon, so no isomers.
Question
What does the E/Z system use to decide?
Answer
**Priority** by Cahn–Ingold–Prelog: the atom of **higher atomic number** is higher priority. Z = higher-priority groups on the same side; E = on opposite sides.
Question
What is a chiral carbon?
Answer
A carbon bonded to **four different** atoms or groups (a stereocentre, often marked C*).
Question
What are enantiomers?
Answer
The **two non-superimposable mirror-image** forms of a molecule that has a chiral carbon (optical isomers).
Question
How do enantiomers differ physically?
Answer
They **rotate plane-polarised light** by the same angle in **opposite directions** (one +, one −); all their other physical properties are identical.
Question
What is a racemic mixture?
Answer
A **50:50 mixture** of the two enantiomers, which shows **no net rotation** of plane-polarised light (the effects cancel).
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Functional groups: classification of organic compounds
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