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IB Chemistry > 10 Organic Chemistry > Flashcards

10 Organic Chemistry Flashcards

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1
Q

nomenclature - number of carbon chains [8]

A

1 - meth
2 - eth
3 - prop
4 - but
5 - pent
6 - hex
7 - hept
8 - oct

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2
Q

nomenclature - type of bonding [3]

A

all single bonds: -an-
one double bond: -en-
one triple bond: -yn-

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3
Q

nomenclature - functional groups

A

alkaline: hydrocarbons = -e
alkyl: alkaline missing one H
hydroxyl: OH = -ol
amine: NH2 = -amine
halo: X: chloro-, bromo-, iodo-
aldehyde (carbonyl): O=C-H = -al
ketone (carbonyl): O=C (not at the end of chain) = -one
carboxyl (carbonyl): O=C-OH = -oic acid
ester: O=C-OR = -oate
phenyl: cyclic group of atoms with the formula C₆H₅= benzene -1 hydrogen atom

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4
Q

classification of alcohols, halogenoalkanes and amines

A

alcohols and halogenoalkanes - based on number of R groups (molecule with either C or H) bonded to the C atom with the functional group
amines - based on number of R groups bonded to N atom of the amino functional group

1 - primary
2 - secondary
3 - tertiary

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5
Q

structural isomers

A

structural isomers - same molecular formula but different structural formula
- similar chemical properties, different physical properties
* alkanes - methane, ethane, propane = only one possible structural formula

  • alkanes and alkynes are unsaturated (ie. 2x or 3x CB bwt adjacent C atoms)
  • alkene and propene - 1 possible structure
  • butene - 3 structural isomers
  • ethyne and propyne - 1 possible structure
  • butyne - 2 structural isomers
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6
Q

properties of different homologous series - bp

A

carbon chain in homologous series of alkanes increases => LDF increases => boiling point increases
* rate of increases in bp initially fast => slower as percentage increase in mass decreases

  • branching => more spherical molecule => reduced SA in contact bwt them => lower bp
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7
Q

properties of different homologous series - solubility in water

A
  • dependent on polarity of functional group and length of C chain
  • lower members of alcohols, amines, aldehydes, ketones, carboxylic acids - all water soluble
  • as length of non-polar carbon chain increases, solubility in water decreases
  • non-polar functional groups (alkanes, alkenes) - insoluble in water. soluble in non-polar solvents
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8
Q

alkanes - reactivity [2]

A

low reactivity
- strong CC and CH bonds
- low polarity
* only readily undergo combustion with oxygen and substitution with halogens in ultraviolet light

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9
Q

alkanes - combustion

A

all hydrocarbons burn in plentiful supply of oxygen => carbon dioxide and water

C-C and C-H bonds are strong but C=O and O-H bonds are stronger => exothermic reaction => alkanes often used as fuels
insufficient oxygen => incomplete combustion => water + carbon monoxide + carbon
* carbon dioxide is not produced

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10
Q

alkanes - substitution reactions

A

alkanes can react with halogens (eg. chlorine) in ultraviolet light
eg. methane with chlorine => chloromethane + hydrogen chloride

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11
Q

alkanes - chlorination of methane

A

chemical bonds may break either heterolytically or homolytically
homolytic fission: each of the two atoms forming the bond retains one of the shared electrons => 2 free radicals
heterolytic fission: both of the shared electrons go to one of the atoms => positive and negative ion

initiation:
- bond bwt 2 halogens (eg. Cl2) is weaker than the C-C or C-H bond in methane => breaks homolytically in UV light

Cl2 -> Cl* + Cl*

propagation:
- free radicals contain an unpaired electron => highly reactive
- when chlorine radicals come into contact with a methane molecule => hydrogen chloride +methyl radical
- (further propagation step, allows chain reaction to occur as the process can repeat itself) methyl radical also very reactive, reacts with another chlorine molecule => product + regenerate chlorine

CH3* + Cl2 -> CH3-Cl + Cl*

termination:
- when two radicals react together

Cl* + Cl* -> Cl2
CH3* + Cl* -> CH3Cl
CH3* + CH3* -> C2H6

  • substitution can be continued even further to produce trichloromethane and tetrachloromethane

overall reaction = free radical substitution
* in this reaction, no hydrogen radicals H* are not formed

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12
Q

alkenes - addition reactions

A

reactive molecules can add across the double bond in an alkene
- double bond (unsaturated - able to form products by chemical addition)
- product formed - C atom bonded by 4 single bonds (saturated - cannot combine with any additional atoms or radicals)

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13
Q

alkenes - uses of addition reactions

A
  1. bromination
    pure bromine = red liquid
    solution = orange/yellow
    bromine + alkene = colorless
    - test for presence of alkene
  2. hydration
    cracking of oil (heavy hydrocarbons broken down into lighter hydrocarbons by means of heat) => ethene
    although fermentation of starch and sugar => ethanol
    industrial ethanol is usually made from the addition of steam to ethene
  3. hydrogenation
    addition of hydrogen to unsaturated vegetable oils => margarine
    - hydrogenation => decrease number of double bonds in polyunsaturated vegetable oils present in margarine => solid at room temp
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14
Q

alkenes - addition polymerization

A

alkenes can undergo addition reactions with themselves => long chain polymers (under certain conditions)

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15
Q

alcohols - combustion

A

ethanol - used as solvent and fuel, combusts completely in plentiful supply of oxygen => carbon dioxide and water
- already partially oxidized => releases less energy than burning alkanes of comparable mass
- can be obtained through fermentation of biomass => mixed with petrol => gasohol => decreases dependence on crude oil

general equation of combustion of alcohols completely in oxygen
CxH(2x+1)OH + (2n-1)O2 -> xCO2 + (x+1)H2O

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16
Q

alcohols - oxidation of ethanol

A

ethanol is readily oxidized with an acidified solution of potassium dichromate (VI)

ethanol -> ethanal -> ethanoic acid

ethanal (unlike ethanol and ethanoic acid) has no hydrogen bonding => low bp

to stop reaction at aldehyde stage, ethanal can be distilled once it has formed
if complete oxidation to ethanoic acid is needed, the mixture can be heated under reflux (condenser => ethanal does not esacpe)

17
Q

alcohols - oxidation

A

primary alcohols
- all oxidized by an acidified solution of potassium dichromate (VI):
- first oxidized to aldehydes
- then to carboxylic acids

secondary alcohols
- oxidized to ketones which cannot be further oxidized

tertiary alcohols
- cannot be oxidized by acidified potassium dichromate (VI) -> no hydrogen atoms attached directly to C atom containing -OH group
* burn readily => can be oxidized
however, burning destroys C chain

18
Q

substitution reactions - halogenoalkanes

A

halogen electronegativity > carbon => halogenoalkanes have a polar bond

reagents (nucleophiles) with a non-bonding pair of electrons are attracted to the electron-deficient carbon in halogenoalkanes => substitution reactions

  • useful in organic synthesis: wide variety of different compounds can be made by changing the nucleophile
19
Q

substitution reactions - benzene

A

delocalization of electrons in benzene rings => extra stability => benzene (unlike other alkenes) does not readily undergo addition reactions, BUT CAN UNDERGO SUBSTITUTION REACTIONS

  • benzene = high electron density => reacts with electrophiles (electron-deficient species formed in the reaction mixture that can accept electron pairs)

eg. benzene + chlorine (in the presence of aluminum chloride - Cl+ ion = electrophile) => chlorobenzene
benzene + nitric acid (in the presence of sulfuric acid - NO2+ ion = electrophile) => nitrobenzene

20
Q

condensation reactions - alcohols and carboxylic acids

A

alcohols undergo nucleophilic substitution reactions with carboxylic acids (esterification) = condensation reaction

  • alcohols + carboxylic acid (in presence of concentrated sulfuric acid) => ester

esters:
pleasant smell
- natural and artificial flavoring in food
- solvent in perfumes and plasticizers (makes polymers more flexible => used to modify properties of polymers)

21
Q

nucleophilic substitution - mechanism

A

primary halogenoalkane - only one alkyl group attached to C atom bonded to halogen

rate = k[halogenoalkane][nucleophile]
* mechanism involves the formation of a transition state which involves both reactants
- molecularity (no. of molecules that come together to react in a single-step reaction) of reaction = 2 => SN2 (bimolecular nucleophilic substitution)

SN2 reaction:
- stereospecific (ie. starting reagents differing in their configuration are converted into stereoisomeric products)
- inversion of configuration at central C atom

tertiary halogenoalkanes (3 alkyl groups attached to C atom bonded to halogen)

rate = k[halogenoalkane]
two-step mechanism:
step 1 - heterolytic fission of C-halogen bond (RDS)
- molecularity of reaction = 1 => SN1 (unimolecular nucleophilic substitution)

hydrolysis of secondary halogenoalkanes (2 alkyl groups attached to C atom bonded to halogen) can occur either by SN1 or SN2 or a combination of both

22
Q

nucleophilic substitutions - solvent

A

protic solvent (ie. hydrogen bonded to an electronegative atom - F, N, O) => polar (eg. ethanol, water) => support breakdown of halogenoalkanes into carbocations and halide ions => favor SN1 reactions

aprotic solvents (no H bonded to electronegative atoms => cannot form hydrogen bonds) => less polar => favor SN2 mechanism involving transition state

23
Q

factors affecting rate of nucleophilic substitution

A
  1. nature of nucleophile
    effectiveness of nucleophile depends on electron density - anions tend to be more reactive than corresponding neutral species
    eg. hydroxide ion is a better nucleophile than water
  2. nature of halogen
    for both SN1 and SN2 reactions (fastest to slowest):
    iodoalkanes -> bromoalkanes -> chloroalkanes
    - because of relative bond enthalpies: C-I bond is the weakest => breaks the most easily
  3. nature of halogenoalkane
    (fastest -> slowest)
    tertiary -> secondary -> primary
    - SN1 formation which involves formation of intermediate carbocation is faster than SN2 reaction involving transition state with relatively high activation energy
24
Q

electrophilic addition - symmetric alkenes

A
  • can occur in the dark => no free radical mechanism
    Eg. ethene + hydrogen bromide (electrophile) -> bromoethane
  • ethene has high electron density above and below plane of molecule
  • greater electronegativity of bromine compared to hydrogen => polar molecule
  • hydrogen atom (with slight +ve charge) from H-Br is attracted to double bond in ethene => H-Br break => bromide ion, hydrogen atom adds to one of the ethene C atoms => other C atom left +ve charge (carbocation)
    carbocation + bromide ion -> bromoethane
  • electrophilic addition can also take place when bromine adds to ethene in a non-polar solvent => 1,2-dibromoethane
  • bromine is non-polar, induced dipole is formed by the electron cloud as it approaches the double bond in ethene
25
Q

electrophilic addition - Markovnikov’s rule

A

when hydrogen halides add to asymmetric alkenes, 2 products are possible depending on which C atom the hydrogen atom bonds to.

Markovnikov’s rule - halogen adds to C atom that is bonded to more R-groups

  • R-groups (alkyl groups) push electrons towards the C atom they are attracted to => stabilizes +ve charge on carbocation (positive inductive effect)
    => halogen adds to more stable carbocation
  • tertiary carbocation (3 R-groups) - most stable
  • secondary carbocation (2 R-groups)
  • primary carbocation (1 R-group) - least stable
26
Q

electrophilic substitution - nitration of benzene

A

benzene + concentrated nitric acid + concentrated sulfuric acid (@ 50°C) -> nitrobenzene + water
* temp beyond 50°C => further nitration to dinitrobenzene

  • concentrated sulfuric acid = catalyst - protonates (adds protons) nitric acid, nitric acid => lose water => electrophile = nitronium ion (NO2 +)
  • in the presence of more acidic sulfuric acid, nitric acid acts as base
  • nitronium ion is attracted to delocalized pi-bond => attaches to one of the C atoms (requires considerable Ea because delocalized pi-bond is partially broken) => +ve charge is distributed over remains of the pi-bond in the intermediate
  • intermediate loses proton and energy is evolved as pi-bond is reformed => proton recombines with hydrogensulfate ion => regenerate catalyst
27
Q

reduction reactions - carbonyl compounds

A

carbonyl compounds: C=O

reducing agents provide effective source of H- ions => act as reducing agents undergoing nucleophilic addition reaction with electron deficient C in the carbonyl group

sodium borohydride - NaBH4:
- used in protic solvents (eg. ethanol, water)
- cannot reduce carboxylic acids

lithium aluminum hydride - LiAlH4:
- stronger RA
- reacts with water => must initially be used in aprotic solvents (eg. ether), reaction is then acidified to obtain product

hydrogen:
- used in presence of nickel / platinum / palladium catalyst

aldehydes -> primary alcohols
ketones -> secondary alcohols
carboxylic acids -> primary alcohols

28
Q

reduction reactions - nitrobenzene

A

nitrobenzene -> phenylamine (2-step reaction)

  1. refluxed with a mixture of tin and concentrated hydrochloric acid. tin provides electrons (RA) => phenylammonium ion
  2. addition of sodium hydroxide solution => release free amine
29
Q

stereoisomerism - cis-trans isomerism

A
  • cis-trans and E/Z isomerism occur when rotation about a bond is restricted or prevented
  • single bond between 2 C atoms => free rotation about the bond is possible
  • double bond (sigma and pi bond), pi-bond formed from combination of two p-orbitals (1 from each C atom)
  • for combination to occur, both p orbitals must be on same plane, rotating the bond => pi-bond break
  • always occur in alkenes when 2 groups (X and Y) attached to each of the C atoms are different
  • cis-isomerism: substituents are on side side of double bond
  • trans-isomerism: substituents on different sides of double bond
30
Q

stereoisomerism - E/Z isomerism

A
  • covers all cases (even when groups are different) where free rotation about a C=C double bond is not possible
  • higher the atomic number of the attached atoms to C atom, the higher the priority
  • highest priorities lie on same side of double bond => Z isomer
  • highest priorities lie on different side of double bond => E isomer
    ** (E)nemies lie on different sides
  • E/Z isomers and cis-trans isomers tend to have similar chemical properties but different physical properties
31
Q

stereoisomerism - optical isomers

A
  • shown by all compounds that contain at least one asymmetric or chiral C atom (stereocenter) (ie. contains 4 different groups bonded to it) => 2 isomers (enantiomer) => mirror images of each other
  • optically active with plane-polarized light (light passed through a polarizing filter => waves vibrate in one plane)
  • both enantiomers rotate about the plane of the plane-polarized light => one rotates to the left, the other to the right
  • identical physical properties
  • identical chemical properties except when interacting with other optically active substances
  • molecules with 2 or more stereocenters => 2 or more different stereoisomers are possible

*enantiomers: mirror images
* diastereomers: not mirror images
- diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent stereocenters

IB Chemistry (5 decks)

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