10 Organic Chemistry Flashcards
nomenclature - number of carbon chains [8]
1 - meth
2 - eth
3 - prop
4 - but
5 - pent
6 - hex
7 - hept
8 - oct
nomenclature - type of bonding [3]
all single bonds: -an-
one double bond: -en-
one triple bond: -yn-
nomenclature - functional groups
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
classification of alcohols, halogenoalkanes and amines
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
structural isomers
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
properties of different homologous series - bp
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
properties of different homologous series - solubility in water
- 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
alkanes - reactivity [2]
low reactivity
- strong CC and CH bonds
- low polarity
* only readily undergo combustion with oxygen and substitution with halogens in ultraviolet light
alkanes - combustion
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
alkanes - substitution reactions
alkanes can react with halogens (eg. chlorine) in ultraviolet light
eg. methane with chlorine => chloromethane + hydrogen chloride
alkanes - chlorination of methane
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
alkenes - addition reactions
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)
alkenes - uses of addition reactions
- bromination
pure bromine = red liquid
solution = orange/yellow
bromine + alkene = colorless
- test for presence of alkene - 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 - 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
alkenes - addition polymerization
alkenes can undergo addition reactions with themselves => long chain polymers (under certain conditions)
alcohols - combustion
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
alcohols - oxidation of ethanol
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)
alcohols - oxidation
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
substitution reactions - halogenoalkanes
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
substitution reactions - benzene
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
condensation reactions - alcohols and carboxylic acids
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)
nucleophilic substitution - mechanism
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
nucleophilic substitutions - solvent
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
factors affecting rate of nucleophilic substitution
- 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 - 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 - 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
electrophilic addition - symmetric alkenes
- 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
