Isomerism

Question 1

What are “structural isomers?”

Answer 1

• Structural isomers have the same molecular formula but a different structural formula.

Question 2

What are the 3 types of structural isomers?

Answer 2

• Chain
• Position
• Functional group

Question 3

What are “chain isomers?”

Answer 3

• Same molecular formula but different arrangement of the carbon skeleton.

Question 4

What are “positional isomers?”

Answer 4

• Same molecular formula but different position of the functional group on the carbon skeleton.

Question 5

What are “functional group isomers?”

Answer 5

• Same molecular formula but different functional group.

Question 6

Give 3 examples of different molecules that would be functional group isomers .

Answer 6

Cycloalkanes — alkane
Aldehydes —- Ketones
Alcohols —- ethers

Question 7

What are “stereoisomers?”

Answer 7

• Same structural formula but different arrangement of atoms in space.

Question 8

True or False

Molecules with single bonds between carbon atoms cannot rotate.

Answer 8

• FALSE
• Molecules can rotate freely around single C-C bonds.

Question 9

Can molecules rotate around C=C double bonds

Answer 9

• No.
• Molecules can’t rotate around C=C double bonds (ie. the atoms around the C=C double bond are in a fixed position.)

() - extra info for clarification.

Question 10

How can you tell if 2 molecules are stereoisomers of each other?

Answer 10

• Can’t twist double bond to turn one isomer into another (as atoms can’t rotate around the C=C double bond.)
• Atoms bonded directly to the double bond are flat/ planar.

Question 11

What terminology do we use to describe how atoms can’t rotate around the rigid C=C double bond?

Answer 11

• We say there is “restricted rotation” around the C=C double bond.

Question 12

When do we get E/Z isomerism?

Answer 12

• When you have two different atoms/ groups of atoms attached to the same carbon.
• Restricted rotation about the planar carbon-carbon double bond.

Question 13

What is meant when we say a molecule is a Z molecule (using E-Z notation?)

Answer 13

• Z: two priority groups are on the same side of the double bond.

Question 14

What is meant when we say a molecule is a E molecule (using E-Z notation?)

Answer 14

E: the priority groups on opposite sides of the double bond.

Question 15

When could we call a molecule cis/ trans?

Answer 15

• Cis: two of same groups on same side of double bond.
• Trans: two of the same groups on opposite sides of double bond.

Question 16

What form of isomerism is E-Z isomerism?

Answer 16

• E-Z isomerism is a form of stereoisomerism.

Question 17

When are CIP priority rules applied to a molecule?

Answer 17

• CIP rules are applied when deciding if an isomer is E or Z when 2 different groups are bonded to the SAME carbon atom.

Question 18

How would you determine if a molecule is an E/Z isomer when there are 4 DIFFERENT groups around the C=C double bond?

Answer 18

• Split molecule in half.
• Work out atomic number of first element DIRECTLY BONDED to C=C double bond –> atom with highest atomic number is given higher priority.
• Two higher priority groups same side of double bond: Z
• Two higher priority groups on opposite sides of double bond: E.

Question 19

What do you do when determing whether molecules are E/Z isomers, 4 different groups around C=C double bond AND element DIRECTLY BONDED to C=C double bond is the same?

Answer 19

• Do normal steps.
• When you reach the two atoms DIRECTLY BONDED to the C=C double bond that have same atomic no, look at the next atom down the chain, if they are still the same, keep going down the chains until one atom’s atomic no is greater than the other atom’s - this will be the higher priority group.

Question 20

What does CIP priority rules stand for?

Answer 20

Cahn–Ingold–Prelog (CIP) priority rules

Question 21

What do ethers look like? What would be the name of an ether with two ethy groups?

Answer 21

• Oxygen in middle of carbon chain with carbons on either side of the chain.
• 2 ethyl groups on either side of the O = Ethoxyethane.

Question 22

How do we name ethers? Prefix/ suffix?

Answer 22

• Prefix: Alkyloxy at the beginning of the name - ie. it would be arranged in alphebetical order (e.g., if there was bromine group, it would be written before an ethyloxy group for EXAMPLE.)

() - extra

Question 23

When does E-Z isomerism actually occur?

Answer 23

• C=C double bond present.
• Two different groups on EACH side of the molecule (the groups don’t have to ALL be different, just different on each side.)

Question 24

When does cis/ trans isomerism occur?

Answer 24

• Will occur when there’s an identical group on either side of the C=C double bond.

Question 25

How should we always display groups attached to carbon atoms?

Answer 25

• Always display them as structural formulae (not molecular!)

Question 26

How many isomers do each of these molecules have:
1.) C₄H₁₀
2.) C₅H₁₂
3.) C₆H₁₄

Gautum said learning these 3 will be useful for the exam.

Answer 26

1.) 2 isomers
2.) 2 isomers
3.) 5 isomers.

Question 27

What does E/Z isomerism occur as a result of?

Answer 27

• E/z isomerism occurs as a result of restricted rotation about the planar carbon - carbon double bond.