carbon-based compounds

Question 1

what are the main characteristics of carbon based compounds?

Answer 1

• always contain C-H
• may contain non-metals (H,O, halogens…)
• covalent bonds
• low melting/boiling points
• can undergo combustion

Question 2

why is life carbon based?

Answer 2

carbon 4 valence electrons allow for:

• very stable bonds (C=C or C-H)
• large versatility in bonding (substitution of H bonds with functional groups to create varying physical and chemical properties)
• chains of infinite length
• several oxidation states

Question 3

describe the hybridisation of carbon

Answer 3

C (1s2 , 2s2 , 2p6 )has 4 valence electrons -> 2s orbital completely occupied, px/y have 1 electron each, pz is empty

Question 4

what type of orbitals react in covalent reactions with carbon and what happens?

Answer 4

C forms 4 energetically equivalent hybrid orbitals (sp3)

• placed further away from each other to decrease repulsion between -ve charges -> creates a tetrahedral arrangement (3D structures allow for the physical/chemical properties of molecules)
• sp3 orbitals overlap with s and p orbitals to form σ bonds
• C is also sp2 (double bond) or sp (triple bond) hybridised
• non-hybridised orbitals form pi molecular orbitals

Question 5

what are functional groups?

Answer 5

groups of atoms/molecules which are added to simple C chains, giving specific properties

Question 6

describe the properties of hydrocarbons

Answer 6

composed of C and H, no functional groups

• saturated/unsaturated
• form linear, branched or cyclic structures
• apolar substances: do not mix with water
  
  • bonds C-C (equal electronegativity, apolar)
  • bonds C-H (very small electronegativity difference, apolar)

Question 7

what is the simplest form of hydrocarbons?

Answer 7

ALKANES: CnH2n+2

• saturated
• meth- (1), eth- (2), prop- (3) but-(4), etc… + suffix -ane

Question 8

why does rotation happen in single bonds?

Answer 8

rotation around single C-C bond is free/unrestricted -> create conformational isomers (modulates their chemical properties/biological function)
- gives different conformations to molecules:
eclipsed when groups are on the same plane
spherical when functional groups are on opposite planes

Question 9

cyclic alkanes

Answer 9

formed by the removal of H bonds to create C-C bonds, very strained bonds (largest the cycle, easier it is to break)
not planar structure: sp3 angles are around 109, making it impossible for hybrid orbitals to obtain (would create very high energy/unstable molecule)

Question 10

what is the preferred structure in cyclic alkanes and why?

Answer 10

CHAIR conformation is preferred: 4 atoms on plane, 1 below:

has a lower relative potential energy compared to other shapes

Question 11

describe the difference between axial and equatorial planes

Answer 11

• when functional group replaces H substituent may have axial(along vertical axis) or equatorial (closer to plane/x axis) orientation
  
  • equatorial is lowest energy (preferred)

Question 12

describe the properties of alkanes with substituents

Answer 12

1 or more H replaced by halogens/another alkyl chain:

• when 4 or more C are present may form structural isomers
• mainly gases (can be liquid or waxing solids)
• methane to butane gases -> heating fuels
• pentane to octane -> volatile liquids (fuels)
• nonane to heptadecane are liquids
• longer the C chain are waxy solids (paraffins) *petroleum jelly is mixture of paraffins
• higher C chain, higher boiling point: side by side interactions increase between molecules (intermolecular interactions)
• branches alkanes have lower boiling points: intermolecular packing is harder due to 3D arrangement (molecules easier to break apart)
• cyclic alkanes have higher boiling points: more rigid structures easier to pack

Question 13

describe the reactions of alkanes

Answer 13

• poorly reactive (C-C bond is difficult to break)
• combustion: alkane + O2 –> CO2 + H2O + energy (reactions with insufficient oxygen lead to CO)
• reaction with H gas form haloalkanes through radical reactions (contain 1 unpaired electron, extremely reactive): UV lights cause homolytic scission of bonds

Question 14

what are the reactions of halogenoalkanes and why are they more reactive than alkanes?

Answer 14

nucleophilic substitution reaction:

electronegativity between C-X bonds polarises the bond (partial negative charge on halogen, partial positive on C)

Question 15

what is a nucleophile?

Answer 15

molecule with affinity to nucleus, containing at least 1 pair in non bonding orbital, attracted to positive C to replace the electrophile (X-)
ex. H2O, NH3, OH-

Question 16

what happens in a SN2 reaction?

Answer 16

nucleophile attacks the carbon from the back, no carbocation intermediateis formed but a transition state (leaving group leaves as nucleophile approaches, causes inversion of configuration - umbrella):

• forms a stable halogen species (negatively charged)
• second order reaction: has a rate limiting reaction (depends on nucleophile and substrate concentration) -> K= [sub][nuc]
• reactivity: methyl > 1ry substrate > 2ry substrate > 3ry substrate
  
  • methyl has not other C atom around it, exposed to attack from nucleophile (lower to highest steric hindrance from adjacent groups)

Question 17

what happens in a SN1 reactions?

Answer 17

formation of intermediate carbocation (possible rearrangements of charged molecules):

• sharing of carbon atoms between nucleophile and electrophile
• first order reaction (only depend on concentration of substrate -> K=[sub]
• inversion can happen but common shape is mixture of original molecule and nucleophile addition