Topic 10: Organic Chemistry Flashcards
affixes for number of carbons
1 = meth- 2 = eth- 3 = prop- 4 = but- 5 = pent- 6 = hex- 7 = hept- 8 = oct-
affixes for bonding
single bond: -an-
double bond: -en-
triple bond: -yn-
affix of alkane
-ane
affix of alkene
-ene
affix of alkyne
-yne
affix of alcohol
-ol
affix of carboxylic acid
-oic acid
affix of ether
R1 = -oxy
R2 = -ane
R2 must be the longest chain (so it’ll be used as the parent name)
affix of halide
chloro-/bromo-/iodo-
affix of aldehyde
-al
affix of ketone
-one
affix of ester
R1: alkyl group name (e.g. methyl, ethyl)
R2: -oate
alkene + water -> ?
alcohol
trends in the alkene homologous series
- increase in b.pt down the homologous series
- increase in strength of Van der Waals/London/dispersion forces
- increase in size of molecule/number of electrons
features of a homologous series
- same general formula
- successive members differ by a CH2 chain
- same functional group
- similar chemical properties
- gradual change in physical properties (e.g. m.pt/b.pt)
catenation
carbon’s ability to link itself to form chains and rings
saturated compounds
contain only single bonds
unsaturated compounds
compounds containing double or triple bonds
aliphatics
- compounds that don’t contain a benzene ring
- can be saturated or unsaturated
arenes
- compounds that contain a benzene ring
- all are unsaturated
electrophile
- electron-deficient species
- attracted to electron-rich parts of molecules
- positive ions or at least have partial positive charge
- act as lewis acids
nucleophile
- electron-rich species
- attracted to parts of molecules that are electron-deficient
- nucleophiles have a lone pair of e-s and may also have negative charge
- act as lewis bases
effect of branching on b.pt
- molecules become more spherical due to branching
- ↓ contact SA = ↓ no. of London dispersion forces = ↓ boiling point
organic compounds with London dispersion forces as their strongest intermolecular force
- alkane
- alkene
- alkyne
organic compounds with dipole-dipole forces as their strongest intermolecular force
- ester
- aldehyde
- ketone
organic compounds with hydrogen bonding as their strongest intermolecular force
- amine
- alcohol
- carboxylic acid
factors affecting solubility in water of organic compounds
- polarity of functional group:
- chain length:
factors affecting solubility in water of organic compounds: chain length
- hydrocarbon chain is non-polar
- and non-polar substance prefer to dissolve in non-polar solvents
- lower members of alcohols, amines, aldehydes, ketones, and carboxylic acids are water soluble
- but as the hydrocarbon chain increases in length, solubility in water decreases
ideal solvent for organic compounds
propan-1-ol
- as it contains both polar and non-polar compounds
- so it can dissolve both types (to some extent)
reactivity of alkanes
- saturated hydrocarbons with strong C-C and C-H bonds
- as C-H and C-C bonds are non-polar, they aren’t susceptible to attack by common reactants
- so alkanes are generally stable under most conditions and can be stored/transported/compressed safely
- only readily undergo combustion reactions with oxygen
- only undergoes substitution rxns with halogens in UV light
combustion of alkanes
- alkanes release a significant amount of energy when broken
- so they are widely used as fuels
- alkane combustion reactions are highly exothermic because of the large amounts of energy released when forming CO2 and H2O
- the products are fully oxidized, so alkane undergoes complete combustion
combustion of hydrocarbons in limited oxygen conditions
- incomplete combustion
- CO and H2O produced instead
- in extreme oxygen limitation, just C and H2O will be produced
combustion of hydrocarbons
- under complete or incomplete combustion depending on oxygen availability
- large amounts of energy are released generally
- as C:H ratio increases with unsaturation, so does the smokiness of the flame due to unburned hydrocarbon
why is the combustion of hydrocarbons a problem?
- CO2 and H2O are greenhouse gases
- thus they contribute to global warming and climate change
- CO is a toxin as it combines irreversibly with blood hemoglobin, preventing it from carrying oxygen
- unburned carbon is released into the air as particulates
- they have a direct effect on human health
- they also catalyze the formation of smog in polluted air
- they are also the source of global dimming
substitution reaction
- main reaction undergone by alkanes
- occurs when another reactant (halogen) takes the place of a hydrogen atom in the alkane
- UV rays needed to provide energy to break the covalent bonds in the halogen molecule
- energy splits the halogen molecule into free radicals
- the radicals start a chain reaction in which a mixture of products (including the halogenoalkane) is formed
homolytic fission
- when a covalent bond breaks by splitting the shared pair of e-s between the 2 products
- produces 2 free radicals, each with 1 unpaired e-
heterolytic fission
- when a covalent bond breaks and the shared pair of e-s go to one product
- produces 2 oppositely-charged ions
free radical substitution
- Initiation:
Cl2 → 2Cl•
- in UV light - Propagation: (formation of new radicals)
CH4 + Cl• → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl• - Termination (when 2 radicals react together)
CH3• + Cl• → CH3Cl
addition reaction
- occurs when 2 reactants combine to form a single product
- characteristic of unsaturated compounds
substitution reaction
- occurs when 1 atom or group of atoms in a compound is replaced by a different atom or group
- characteristic of saturated and aromatic compounds
addition-elimination reaction
- AKA condensation reaction
- occurs when 2 molecules join together (addition) and in the process small molecules are lost (elimination)
- reaction occurs between a functional group in each reactant
addition reaction (alkene + H2)
alkane
addition reaction (alkene + hydrogen halide)
halogenoalkane
addition reaction (alkene + halogen)
dihalogenoalkane
addition reaction (alkene + water)
alcohol
catalyst: H2SO4
uses of addition reactions
- bromination
- hydrogenation
- hydration
uses of addition reactions: bromination
- reaction used to distinguish between alkanes and alkenes
- bromine water changes from brown to colorless
- for alkanes: occurs in UV light only
- for alkenes: will occur rapidly at room temp
uses of addition reactions: hydrogenation
- used in margarine industry
- converts unsaturated hydrocarbon chains into saturated compounds with higher melting points
- so that margarine is solid at room temp
uses of addition reactions: hydration
- can be used to form an important solvent (ethanol)
- addition of steam to ethene
addition polymerization reactions
- under certain conditions, alkenes can undergo addition reactions with themselves
- to form a long chain polymer made up of thousands of C atoms
- can also be extended to other substituted alkenes to give a wide variety of different addition polymers
reactivity of alcohols
as the -OH group is polar, it increases alcohols’ solubility in water relative to alkanes
combustion of alcohols
- like hydrocarbons, alcohol combustion produces significant amounts of energy along with CO2 and H2O
- the amount of energy released per mole increases down the homologous series
incomplete combustion of alcohols
like alkane, the incomplete combustion of alcohol produces CO and H2O instead of CO2 and H2O
oxidation of alcohols
- combustion completely oxidizes alcohol molecules
- but alcohols can also react with oxidizing agents that selectively oxidize the O atom in the -OH group
- this keeps the carbon skeleton intact
- allows alcohols to be oxidized into other organic compounds
- the most common oxidant used as acidified potassium dichromate (VI) [K2Cr2O7], and the bright orange solution is reduced to just green Cr (III) over the course of the rxn
- oxidants are often represented as + [O] in diagrams
oxidation of primary alcohols
- primary alcohols are oxidized to form aldehydes, then further oxidized to form carboxylic acids
- typically oxidized by acidified K2Cr2O7 (orange –> green)
- the second oxidation (aldehydes to carboxylic acids) needs to be heated under reflux
why can’t wine be left exposed to air?
- bacteria will slowly oxidize the ethanol to ethanoic acid
- i.e. alcohol to carboxylic acid
obtaining aldehydes from primary alcohols
- the oxidation of primary alcohols can be stopped on the first step of oxidation (alcohols to aldehydes)
- distillation can be utilized to remove the aldehydes from the reaction mixture
- this is possible because aldehydes have lower boiling points than alcohols and carboxylic acids
obtaining carboxylic acids from primary alcohols
- simply leave the aldehyde alone with the oxidant for an extended period of time
- preferably heated under reflux
oxidation of secondary alcohols
- secondary alcohols are oxidized to ketone
- preferably heated under reflux
- typically oxidized by acidified K2Cr2O7 (orange –> green)
oxidation of tertiary alcohols
- tertiary alcohols don’t readily oxidize
- their carbon structure needs to be broken and this requires a lot more energy than the oxidation of other alcohols
- so unlike the other 2 oxidations, oxidizing tertiary alcohols with K2Cr2O7 won’t result in a color change as there is no reaction
esterification reaction
carboxylic acid + alcohol → ester + water
- reversible reaction
- type of condensation reaction
- catalysed by concentrated H2SO4
- the acid becomes the -oate while the alcohol becomes the alkyl group
separating and distinguishing esters in an esterification reaction
- as the ester has the lowest boiling point, they can be removed via distillation
- the presence of esters can be identified by their distinct fruity, sweet smell
- as they don’t have -OH groups (unlike the reactants), they can’t form hydrogen bonds and will remain as an insoluble layer on the surface of water
structure of halogenoalkanes
- basically like alkanes except 1 halogen atom is substituted for 1 of the hydrogen atoms
- as halogenoalkanes are saturated molecules, they undergo substitution reactions
reactivity of halogenoalkanes
- halogenoalkanes contain a polar bond
- this makes them more reactive than alkanes
- the halogen atom is more electronegative than the C atom
- so they exert a stronger pull on the shared e-s in the C-H bond
- thus halogen exerts a partial -tive charge while the C atom exerts a partial +tive charge
- the C atom is then said to be e- deficient
- this e- deficient C atom defines much of a halogenoalkane’s reactivity
organic importance of nucleophilic substitution
allows organic compounds to be converted into many other classes of organic compounds
why do carboxylic acids have higher boiling points than alcohols?
- both carboxylic acids and alcohols form H bonds as well as dipole-dipole interactions and London dispersion forces
- however, H bonding can occur between 2 carboxylic acids twice (as their functional group have H and OH)
why is benzene unusual among unsaturated compounds?
- it’s an alkene that’s less saturated than alkynes (its C:H ratio is 1:1)
- it has no isomers
- it doesn’t like addition reactions
importance of benzene
used in the synthesis of drugs, dyes, and plastics
what makes benzene such a stable compound?
- all C atoms are sp2 hybridized
- at “ring level” they form sigma bonds of 120°
- each C contains one unhybridized p orbital that spread out evenly and overlap to form pi bonds above and below “ring level”
- this forms delocalized pi e- clouds with e- density concentrated in donut-shaped rings parallel to the plane of the ring (above and below)
- this lowers the molecule’s internal energy, making it more stable
bond lengths of benzene
- all C to C bond lengths are equal (both - and = bonds)
- each bond contains a share of 3 e-s between the bonded atoms
why is benzene’s ΔH(hyd) lower than standard enthalpy values would suggest?
- benzene is very stable…
- delocalization of e-s minimizes repulsion
- this lowers internal energy by the amount of difference calculated between the actual ΔH(hyd) value and the theoretical value
- this effect is called resonance energy/stabilization energy
why does benzene prefer substitution reactions despite being unsaturated?
- addition reactions would disrupt the delocalized pi e- clouds
- this would involve supplying the resonance energy needed to disrupt the cloud
- plus without the cloud, the product would be less stable
- substitution reactions would preserve the delocalized e- clouds and therefore retain stability
why does benzene have no isomers?
- benzene is symmetrical with alternating single and double bonds
- so all adjacent positions in the ring are equal
SN1
- nucleophilic substitution unimolecular
rate = k [halogenoalkane] - undergone by tertiary halogenoalkanes
- forms carbocation intermediate before forming final product
- tertiary halogenoalkanes have 3 alkyl groups attached to the central C atom, which causes steric hindrance (this makes it difficult for nucleophiles to attack)
- so instead, the halogenoalkane typically detaches the halide heterolytically, leaving an opening for attack
- in the carbocation intermediate state before the nucleophile attacks, the 3 alkyl groups have a stabilizing (positive inductive) effect to help the carbocation remain in the unstable state before being attacked
- in total forms 2 transition states (between reactant –> carbocation intermediate, then between carbocation intermediate –> product)
SN2
- nucleophilic substitution bimolecular
rate = k [halogenoalkane][nucleophile] - undergone by primary halogenoalkanes
- forms unstable transition state before forming final product
- stereospecific: the geometric arrangement of the product is the inverse of the reactant
- favored by polar, aprotic solvents i.e. polar solutions that can’t form H bonds (e.g. propanone)
NOTES: how to tell if an organic compound is primary/secondary/tertiary
- look at the C atom attached to the functional group
- the number of C atoms attached to it tells you whether it’s primary/secondary/tertiary (e.g. the C atom attached to a primary alcohol’s functional group has a bond with one C atom)
NOTES: how to tell if an organic compound is chiral
- if a chiral carbon is present, the compound is chiral
- a chiral carbon is a C atom with 4 different groups attached
- (CH3)3 does NOT count as “different groups”
factors affecting SN rates
- effect of mechanism (SN1 vs SN2): SN1 is faster, so rate of reaction: tertiary > secondary > primary
- secondary alcohols undergo a mix of SN1 and SN2
- polarity of C-X bond: more polar = the C is more e- deficient = more vulnerable to attack (F > … > I)
- strength of C-X bond: weaker bond = easier to overcome = more vulnerable to attack (I > … > F)
- effect of strength of bond > effect of polarity of bond, so iodoalkanes are generally favored for faster SN reaction rates
electrophilic addition reaction
- undergone by alkenes
-
characteristics of alkenes that allow them to undergo many synthetic pathways
- the atoms of the C=C bond are sp2 hybridized
- planar triangular bond geometry with angle of 120
- pi bond creates 2 areas of electron density (one above and one below the plane of the bond axis)
- pi bond e-s are attractive to electrophiles
- the pi bond e-s are much weaker than the sigma bond e-s so they’re much more easily broken during addition
mechanism of ethene + Br2
- bromine becomes polarized by the e- rich region of the alkane (due to e- repulsion)
- Br2 splits heterolytically to form Br+ and Br-
- Br+ attacks the pi bond and breaks it, bonding with a C atom
- the other C atom is left with an incomplete octet, forming an unstable carbocation intermediate
- it reacts with the Br- to form 1,2-dibromoethane
mechanism of ethene + HBr
- HBr is already polar so it doesn’t get polarized by the e- rich region of the alkane
- HBr splits heterolytically to form H+ and Br-
- H+ attacks the pi bond and breaks it, bonding with a C atom
- the other C atom is left with an incomplete octet, forming an unstable carbocation intermediate
- it reacts with the Br- to form bromoethane
considering unsymmetric electrophilic additions (mechanism of propane + HBr)
- 2 possible pathways that produce isomers of bromopropane
- remember that the preferred pathway is the one that gives a more stable carbocation intermediate
- also remember that alkyl groups around a C atom give a stabilizing effect
- tip: H+ will bond to the C atom that is already bonded to the greater no of Hs
why does benzene undergo electrophilic substitution?
because its highly stable aromatic ring prefers substitutions, but its e- dense regions are attractive to electrophiles
electrophilic substitution mechanism
slow rxn: benzene + E+ –> non-aromatic carbocation with the +tive charge distributed over the molecule
fast rxn: non-aromatic carbocation –> benzene with E + H+
- the loss of the H+ ion restores the product’s electrically neutral state
- because 2 e-s from the C-H bond move to regenerate the aromatic ring
- the product of an electrophilic substitution is always more stable than the reactant
nitrating mixture
- mixture of conc H2SO4 and conc HNO3
- as the stronger acid, H2SO4 protonates HNO3 to H2NO3+
- H2NO3+ then breaks down to produce H2O + NO2 +
- this is used to supply NO2 + for the nitration of benzene
nitration of benzene
- electrophilic substitution that adds -NO2 in place of -H
- the NO2 is generated using a nitrating mixture
CONDITIONS: heat at 50°C with conc H2SO4 (part of the nitrating mixture) - the released H+ reforms HSO4 - back to H2SO4
reduction of carbonyls
- carbonyl compounds: those with the C=O group (i.e. alcohols, ketones, aldehydes, and carboxylic acids)
- their oxidations can be reversed using NaBH4 (aqueous/alcoholic solutions) or LiAlH4 (anhydrous conditions)
when should NaBH4 or LiAlH4 be used?
- NaBH4 is generally preferred as it’s safer
- but LiAlH4 is used where NaBH4 is not strong enough (with carboxylic acids)
reduction with NaBH4
heat aldehydes/ketones in the presence of NaBH4
reduction with LiAlH4
- heat carboxylic acids in the presence of LiAlH4 and dry ether
- the rxn can’t be stopped at the aldehyde stage and will proceed until the primary alcohols are synthesized
- this is bc aldehydes react too readily with LiAlH4
reduction of nitrobenzene
- React C6H5NO2 with Sn and HCl to form C6H5NH3 +
2. React C6H5NH3 + with NaOH. The OH- group will remove the additional H to form H2O and C6H5NH2 (phenylamine)
cis isomers
isomers with the same groups on the same sides of the bond plane
trans isomers
isomers with the same groups on opposite sides of the bond plane
E isomers
isomers with the two highest priority groups on opposite sides of the bond plane
Z isomers
isomers with the two highest priority groups on the same sides of the bond plane
Cahn-Ingold-Prelog rules of priority
- for atoms: higher atomic no = higher priority
Br > Cl > F - for alkyl groups: longer chain = higher priority
C3H7 > C2H5 > CH3
characteristics of chiral compounds
- asymmetric/stereocentre
- can be arranged in 2 different 3D configurations that are mirror images of each other (optical isomerism)
enantiomers
- optical isomers
- i.e. mirror images of each other
- they have opposite configurations at all their chiral centres
racemate
AKA racemic mixture
- mixtures containing equal amounts of 2 enantiomers
diastereomers
- many chiral compounds have more than one chiral centre
- enantiomers are when optical isomers have opposite configs at every chiral centre
- diastereomers are when isomers have opposite configs at more than one (BUT NOT ALL) chiral centre
- therefore they are not mirror images of each other
properties of diastereomers
unlike enantiomers, they differ in physical and chemical properties
properties of enantiomers
- optical activity: when a beam of plane-polarized light is passed through a solution containing enantiomers, the plane of the polarized light is rotated by a certain angle
- a pair of enantiomers will react differently with a common reactant (e.g. a single enantiomer of another compound)
polarimeter
- measures the amount and direction of rotation when a beam of plane-polarized light is passed through a solution of enantiomers
- ordinary light is passed through a polarizer, then through the solution, then through an analyzer
- the results are used to compare different solutions
- separate solutions of enantiomers with similar concentrations will rotate plane-polarized light in equal amounts but in opposite directions
- thus, a racemic mixture is optically inactive
why is the differing reactivity of a pair of enantiomers to other chiral compounds significant?
- biological systems are chiral compounds
- e.g. one enantiomer in a drug can be therapeutic while the other can cause malformations in a foetus
- this resulted in the invention of asymmetric synthesis, a process for the manufacture of a single enantiomer using a chiral catalyst
