Organic Chemistry & Carbon Bonding

Question 1

What do the properties of organic molecules depend on?

Answer 1

Their structure.

Question 2

Why carbon?

Answer 2

• can form strong bonds with many other elements and itself (high bond enthalpies)
• rare ability to catenate (long chains of carbon), allows vast range of possible structures

Question 3

Give a limitation of Lewis structures.

Answer 3

-they do not explain the geometry of the molecules, how the bonds form, or how e- are shared between the atoms.

Question 4

Define atomic orbital theory.

Answer 4

• region within an atom that can hold up to 2 e- with opposite spins.
• 4 types of orbital with specific shapes.
• we cannot know where an e- is at any one time, just use mathematical equations that show where the e- could possibly be.

Question 5

Define px orbitals.

Answer 5

P orbitals that overlap head on are px orbitals that form a sigma bond, those that overlap sideways are py orbitals and form pi bonds. These combined forms a double bond.

Question 6

Define a pi bond.

Answer 6

The pi bond is formed by the overlap of the unhybridized py orbital.

Question 7

Define homologous series.

Answer 7

• family of chemicals with the same chemical group
• similar chemical properties
• each successive member differs by 1 carbon atom

Question 8

Define alkanes.

Answer 8

• general formula CnH2n+2
• saturated, single bonds only.
• each carbon atom has tetrahedral geometry with bond angles of 109.5 degrees.
• non-polar.
• very unreactive: saturated, non-polar, hydrophobic. But are very good fuel sources.

Question 9

Define fractional distillation of crude oil.

Answer 9

• complex mix of hydrocarbons.
• allows the separation of liquids depending on their boiling points.
• as chain length increase so does boiling point.
• short chains are gases, longer chains are liquid and even longer chains are solid.

Question 10

Give & describe the type of isomerism exhibited by alkanes.

Answer 10

• exhibit structural isomerism through branching.

- structural isomers are molecules with the same molecular formula but with different structural arrangements.

Question 11

Define alkanes as cyclic molecules.

Answer 11

• all of the C atoms in the ring are single bonded to other atoms
• still saturated hydrocarbons
• general formula CnH2n

Question 12

Defines alkenes.

Answer 12

• general formula CnH2n
• unsaturated, double bond(s).
• more reactive than alkanes because the pi bond is weaker than a sigma bond.
• suffix = -ene

Question 13

Give & define the type of isomerism exhibited by alkenes.

Answer 13

stereosiomerism

• free rotation around single bonds
• the double bond does not allow this
• stereoisomers are molecules with the same structural formula but with different arrangements of atoms in space.
• cis = heaviest groups are on same side of double bond
• trans = heaviest groups are across the double bond

Question 14

Define alkynes.

Answer 14

• general formula CnH2n-2
• functional group is a triple bond (sigma + 2 pi)
• more reactive than alkenes because 2nd pi bond s even weaker than the first.

Question 15

Define the effects of structural isomerism with organic halides.

Answer 15

• chemical behaviour is different.

- physical properties are usually different.

Question 16

Define the effects of stereoisomerism with organic halides.

Answer 16

-physical properties are usually different.

Question 17

Define the effects of optical isomerism with organic halides.

Answer 17

• molecules with chiral centres can exist as two optical isomers.
• mirror images of each other & are non-superimposable.
• rotate plane-polarised light in different direction (left or right).
• optical isomers are 2 molecules with the same chemical & structural formulae but different spatial arrangements of the atoms.

Question 18

Define chiral centre.

Answer 18

carbon atom bonded to 4 different groups.

Question 19

Explain why optical isomers have different biological properties & give an example.

Answer 19

• biological systems are very selective in term of the enantiomers they can tolerate.
• biological receptors often respond differently to different enantiomers of a compound.
• eg receptors in the nose are chiral, so enantiomers have different smells.

Question 20

How do alcohols exhibit structural isomerism?

Answer 20

• primary alcohols have highest boiling points.

- primary alcohols are most easily oxidised. Tertiary alcohols cannot be oxidised at all.

Question 21

What type of isomerism do all amino acids except glycine exhibit?

Answer 21

optical stereoismerism.

Question 22

What is the structure of benzene?

Answer 22

Flat planar hexagon.

Question 23

If benzene had alternating single & double bonds what would we expect?

Answer 23

To see different bond lengths.

All bonds are actually 1.40 A.

Question 24

Why are highly conjugated molecules often coloured?

Answer 24

They often absorb light in the visible region.