topic ten Flashcards
what is catenation
the ability to form covalent bonds with itself, producing long chains and rings
homologous group
same general formula and functional group
In summary, a homologous series is a group of organic compounds that:
contain the same functional group,
have the same general formula,
differ by a CH2 group,
have similar chemical properties,
show a gradation in physical properties such as boiling point.
In full structural formulae, which bonds are shown
all
condensed structural forumlea
in which the bonds between atoms are omitted
skeletal formula
all atoms are omitted leaving only the backbone of the molecule.
sterochemical ormula
wedges and broken lines
isomers
same molecular formula but different arrangements of atoms
structural isomers
compounds with the same molecular formula but different arranagements of atoms
what categories can structural isomers be divided into
chain isomerism, position isomerism and functional group isomerism
functional groups
single atoms or groups of atoms that give organic compounds their characteristic properties
what is the bronsted lowry thing
donating or accepting a proton
waht is the lewis thing
donating or accepting an electron pair
what does iupac do
an international organisation that aims to establish a consistent system of nomenclature (naming) for organic compounds.
Alcohols that contain two hydroxyl functional groups are called
diols
ethers are made up of
two alkyl groups bodned by an oxygen atom
volatility of ethers
voltatile solvents beacaues theyre unable to form hydrogen bonds between molecules because they lack an oxygen atom directly bonded to a hydrogen atom
amines can be primary, secondary, teritary depening on
how many alkyl groups are bonded to the nitrogen atom of the amine group. Primary amines have one alkyl group, secondary amines have two and tertiary three.
The measure of how easily a substance evaporates is referred to as
its volatility
A highly volatile substance has a
low boiling point and evaporates easily
what are the three factors that determine voltaility
boiling point (increases with increasing molar mass as more intermolecular forces)
branched or not, branched chain isomers usually have lower boiling points than the corresponding straight chain isomers as branching produces a more spherical shape which results in less surface contact. Therefore, branched-chain isomers have weaker intermolecular forces and, consequently, lower boiling points
polar functional groups result in stronger dipole dipole interactions and therefore higher boiling points
how are dimers formed
through hydrogen bonds formed between two carboxyl groups on different molecules
compounds that are able to form hydrogen bonds with water molecules will be soluble in
water
as the length of the hydrogcarbon chain icnreases,
solubility decreases
hydrophobic character increases alongside
size of hydrocarbon chain
what is a phenyl functional group
benzene ring (aromatic compounds contain a benzene ring)
evidence for benzene
all carbon to carbon bond lengths are identical
only one isomer exists and there should be more if benzene consisted of alternating single and double bonds
enthalpy change for hydrogenation of benzene is less exothermic than expected
benzene takes part in substitution reactions rather than
addition
The enthalpy change for the hydrogenation of benzene is less than three times the value for the
hydrogenation of cyclohexene.
why are alkenes relatively unreactive
The carbon–hydrogen bonds are considered to be non-polar (or weakly polar) bonds, because of the small difference in electronegativity of 0.4 units.
The carbon–carbon and carbon–hydrogen bonds are relatively strong covalent bonds. This means that the alkanes are kinetically stable unless supplied with enough energy to overcome the energy barrier (the activation energy) for the reaction.
what is a saturated compound
one that has only carbon carbon single bonds
what is an unsatured compound
one that has double or triple bonds
alkanes are used as fuels because
they release large amounts of heat and are used extensively as fuels
If the oxygen supply is limited, however, the products are either
carbon monoxide (CO) and water, or solid carbon (also known as soot) and water.
what undergoes free radical substitution
alkanes with halogens
requirements for free radical sub
uv radiation
what is a substition reaction
involves the replacement of an atom or group of atoms with another atom or group. In the reaction between an alkane and a halogen, a hydrogen atom is replaced by a halogen atom such as chlorine.
why is free radical known as photochemical reactions
because they only take place in the presence of uv radiation
three steps of free radical sub
initation
propagation
termination
The initiation, propagation and termination steps for the reaction of methane and chlorine
when does homolytic bond fission occur
occurs when a bond breaks evenly, with each atom taking one electron from the bond. This results in the formation of radicals, or free-radicals, highly reactive species with unpaired electrons.
why are alkenes more reactive than alkanes
because of the double bond
what is hydrogenation `
In a hydrogenation reaction, gaseous hydrogen (H2) is added across the carbon-carbon double bond. The product of the reaction is an alkane, a saturated hydrocarbon
conditions for hydrogenation
high pressure and a nickel catalyst
how is hydrogenation used by the margarine industry
to convert veg oil containing unsat hydrocarbon chains to more saturated compounds with higher melting points
hydration of an alkene involves
the reaction of an alkene with steam in the presence of a sulfuric acid (H2SO4) or phosphoric acid (H3PO4) catalyst to produce an alcohol
how is hydration used
This reaction is used in industry to produce alcohols such as ethanol and butan-2-ol from alkenes.
what is halogenation
Alkenes react with halogens to produce dihalogenoalkanes. Such reactions take place readily at room temperature and are indicated by the decolourisation of the reacting halogen. The halogen atoms bond across the carbon-carbon double bond
how to test for alkenes
Bromine, Br2 (l), is added to both samples, which are then shaken. At room temperature, the brown colour of the bromine is immediately DECOLOURISED by the alkene. The mixture of bromine and the alkane shows no colour change. Note that the test for unsaturation can also be carried out with bromine water, Br2(aq), in which case a different product is formed.
BROWN TO COLOURLESS
what is addition polymerisation
Individual alkene molecules (monomers) can bond together to form long chains which are known as polymers.
summarisation of a polymerisation reaction
what is a repeating unit
A repeating unit is the part of a polymer chain whose repetition would produce the complete polymer chain, except for the end-groups.
PVC uses
one of the world’s most important plastics, with a wide range of uses including construction materials, packaging and covers for electrical cables. However, its widespread use is somewhat controversial because its synthesis is associated with toxic by-products called dioxins.
formation of the polymer PVC
formation of PTFE
PTFE uses
This polymer is used extensively for its non-adhesive surface properties. It is widely used in non-stick cooking pans, and is also part of waterproof and breathable fabrics such as Gore-tex®.
uses of PS
insulation, packaging
use of PP
crates and boxes, plastic rope
how is ethanol made from alcohol
bioethanol made by the combustion of alcohol
alcohol is oxidised to produce
ethanoic acid, main component of vinegar
The presence of the hydroxyl functional group makes alcohols susceptible to oxidation in the presence of a suitable oxidising agent. Suitable oxidising agents include
potassium manganate(VII) and acidified potassium dichromate(VI) (Cr2O72−/H+)
the dichromate ion forms an orange solution which changes to a ?? colour when it is reduced ot the Cr3+ ion
green
An alternative oxidising agent is acidified potassium manganate(VII) solution which changes colour from purple to ??? as the manganate(VII) ion, MnO4-, is reduced to the manganese(II) ion, Mn2+.
colourless
The partial oxidation of a primary alcohol produces an
aldehyde
If the aldehyde is the desired product, it can be removed as it forms by
distillation
This is possible because the boiling point of the aldehyde is lower than that of both the alcohol and the carboxylic acid. As the aldehyde evaporates and rises up the distillation column, it passes through the condenser where it condenses back to a liquid and runs down into the flask.
the complete oxidation of a primary alochol produces…
a carboxylic acid
heat under reflux with an excess of the oxidising agent
oxidation of secondary alcohol
ketone wtih acidified potassium dichromate
do tertiary alcohols undergo oxidation reactions
no
when are esters formed
in reactions between alcohols and carboxylic acids
what happens in an esterification or condensation reaction
two smaller molecules combine to form a larger molecuels (an ester) resulting in the loss of a molecule of water
A carboxylic acid and an alcohol react to form what
a carboxylic acid and an alochol
Esters are named from the alcohol and carboxylic acid from which they are formed. The name of the alcohol changes to
the alkyl group (such as ethyl, methyl, etc.). This is followed by the name of the carboxylic acid, with the ending changed to -oate.
uses of esters
Esters are volatile organic compounds with distinctive aromas. For this reason, they are used in synthetic flavours, perfumes, and cosmetics. For example, 3-methylbutyl ethanoate (Figure 14) has an aroma similar to bananas and pears. Certain esters are also used as solvents in lacquers, paints and varnishes.
what are halogenoalkanes
compounds that have one or more hydrogen atoms replaced by a halogen atom
Halogenoalkanes undergo ??? reactions in which an atom or group of atoms are replaced by another atom or group
substitution
what is a nucleophile
an electron-rich species with a lone pair of electrons that is attracted to regions of positive charge.
what type of bond fission in halogenoalkane substition reactions
heterolytic bond fission with the halogen atom taking both the bonding electrons.
In heterolytic bond fission,
a bond breaks with one atom in the bond taking both of the bonding electrons. This results in the formation of oppositely charged ions.
why is benzene stable
due to the delocalised electrons within its structure
why does benzene undrdergo substitution reactions
If benzene were to undergo addition reactions, it would result in the disruption of the ring structure and the loss of stability of the molecule. Therefore, benzene undergoes substitution reactions in which one atom (in this case a hydrogen atom) is replaced by a different atom. This does not cause disruption of the ring structure and is more energetically favourable for the benzene molecule. The delocalised ring of electrons, which represents an area of electron density (Figure 1), is also the site of reactivity. This region of electron density is attractive to species which are known as electrophiles.
electrophiles
species that are electron deficient, and as such, they have either a positive charge or a partial positive charge
The nitration of benzene.
An electrophilic substitution reaction involves the
replacement of a hydrogen atom with an electrophile.
The chlorination of benzene. ( or any halogen)
The equation for the complete combustion of benzene
2C6H6 (l) + 15O2 (g) → 12CO2 (g) + 6H2O (l)
What is the product when chlorine (Cl2) reacts with benzene in the presence of an aluminium chloride (AlCl3) catalyst?
C6H5Cl
what are the two types of nucleophilic substitution
SN1 (substitution nucleophilic unimolecular) and SN2 (substitution nucleophilic bimolecular)
what does a nucleophilic substition reaction involve
A nucleophilic substitution reaction involves the replacement of an atom or group of atoms with another atom or group of atoms. A nucleophile is a species with a lone pair of electrons or a negative charge that is attracted to a region of positive charge (nucleophiles are also Lewis bases).
why is the carbon bonded to the halogen in a halogenoalakane open to attack by a nucleophile
its electron-deficient because of the difference in electronegativity between carbon and a halogen such as chlorine
Each of these nucleophiles will replace the halogen atom, which leaves as a halide ion (known as the
leaving group)
which halogenoalkanes undergo SN2 reactions
primary
is sn2 one step or two step
one step
describe sn2
It is a one-step mechanism in which the nucleophile attacks the electron-deficient carbon atom in what is known as a ‘back-side attack’. The attack of the nucleophile is coupled with the departure of the halide ion, which takes place simultaneously in this one-step process.
what does bimolecular refer to
the molecularity of the rate determining step
what is back side attack
a term used to refer to the approach of the nucleophile to the electron-deficient carbon from the opposite side of the leaving group.
steps of sn2
formation of an unstable transition state in which both the nucleophile and the halogen atom are weakly bonded to the carbon atom; note that these partial bonds are represented by dashed lines. The carbon-to-halogen bond then breaks heterolytically, with the halogen atom taking both the bonding electrons. This results in the formation of a halide ion, X− (the leaving group). The product formed is a primary alcohol.
why is the sn2 reaction described as being stereospecific
The backside attack of the nucleophile causes the inversion of the spatial arrangement of the other groups or atoms around the carbon atom.
A stereospecific reaction is one in which there is only one possible
stereochemical outcome
what do the reaction kineticds of the sn2 mechanism show
the rate of the reaction depends on the concentration of both the halogenoalkane and the nucleophile – the rate-determining step is bimolecular.
does the transition state show up high or low on the energy profile
highest point
The energy level profile for the reaction above is shown in Figure 4. Note the formation of the high energy transition state, which represents a halfway stage in the reaction. At this point, covalent bonds between the nucleophile and the halogen atom are simultaneously being broken and made.
which halogenoalkanes undergo sn1 reactions
tertiary
is sn1 one step or two step
two step
describe sn1
The SN1 mechanism is a two-step reaction and the first step of the reaction involves the breaking of the carbon–halogen bond. This bond breaks heterolytically, with the halogen atom taking both the bonding electrons, resulting in the formation of a carbocation intermediate. Figure 5 shows the first step in the mechanism for the reaction between 2-bromo-2-methylpropane (a tertiary halogenoalkane) and sodium hydroxide. Note the use of a curly arrow to show the movement of a pair of electrons from the C–Br bond to the bromine atom, and the formation of the carbocation when the bond breaks heterolytically. The carbocation formed has a trigonal planar arrangement with the carbon atom being sp2 hybridised.
In the second step of the reaction, the nucleophile uses a lone pair of electrons to form a bond with the carbocation intermediate (Figure 6). Once again, this is shown by the use of a curly arrow from the lone pair of electrons on the nucleophile (the hydroxide ion) to the electron-deficient carbon atom. The compound formed is the tertiary alcohol 2-methylpropan-2-ol.
The ‘1’ in SN1 refers to the ??? of the rate-determining step, which is unimolecular.
molecularity
the reate of reaction of sn1 depends on
the concentration of the halogenoalkane only
which step ahs the highest activation enerfy in sn1
the first step. The second step has the lower activation energy because of the attraction between the oppositely charged ions.
what is steric hinderance
The three bulky methyl (CH3) groups bonded to the carbon atom in the tertiary halogenoalkane make it difficult for the incoming nucleophile to attack from the backside
what is the positive inductive effect
The carbocation is stabilised by each of these methyl groups having an electron-donating effect
SN1 are favoured by waht type of solvent
protic polar
SN2 reactions are favoured by
aprotic polar solvents
Unlike polar protic solvents, polar aprotic solvents cannot form hydrogen bonds with the nucleophile. This maintains the reactivity of the nucleophile, favouring the SN2 mechanism.
Factors affecting the rate of nucleophilic substitution
The classification of the halogenoalkane.
The nature of the nucleophile
The leaving group.
how does The classification of the halogenoalkane affect rate of nuc. sub
Tertiary halogenoalkanes react faster via the SN1 mechanism than primary halogenoalkanes do via the SN2 mechanism. The order in terms of rate of reaction (fastest first) is:
3o > 2o > 1o
how does the nature of thenucleophile affect rate of nuc sub
The effectiveness of a nucleophile depends on its electron density; negatively-charged anions tend to be more reactive than neutral species. This explains why the hydroxide ion is a better nucleophile than a water molecule. The hydroxide ion has a negative charge, whereas the water molecule only has a negative dipole.
how does the leaving group affect rate of nuc sub
The iodide ion is the best leaving group of the halide ions. This is due to the relative bond enthalpies: the C–I bond (228 kJ mol−1) is weaker than both the C–Br bond (285 kJ mol−1) and the C–Cl bond (324 kJ mol−1). In terms of rates of reaction, iodoalkanes react faster than bromoalkanes, which react faster than chloroalkanes.
hat do sn1 reactions begin and end with
SN1 reactions begin with a single enantiomer and result in the production of a racemic mixture containing equal amounts of two enantiomers.
what do sn2 reactions begin and end with
SN2 reactions begin with a single enantiomer and produce only one enantiomer which means that the final product is optically active.
what is a racemic mixture known as
optically inactive
what do electrophilic addition reactions invovle
addition of atoms or groups of atoms across the carbon carbon double bond
what is a double bond composed of
one sigma bond and one pi bond
During an addition reaction, which bond is broken
the weaker pi bond is broken and the atoms that make up the electrophile bond with the two carbon atoms that make up the carbon=carbon double bond.
why are alkenes more reactive
because of the high electron density of the double bond which is attractive to species known as electrophiles
an electrophile is either
positively-charged or electron-deficient
why are there two possible products when hydrogen halides undergo reactions with an asymmetric alkene
major or minor depending on which carbon the hydrogen boinds to
markovnikovs rule
states that when hydrogen halides add to asymmetric alkenes, the hydrogen atom bonds to the carbon atom that is already bonded to the greatest number of hydrogen atoms.
markovnikov why
order of carbocation stability
3o > 2o > 1o
interhalogens
compounds composed of two or more different halogen atoms
The delocalised pi system, with its region of high electron density, makes the benzene molecule susceptible to attack by
electrophiles
when benezene undergoes elec sub what happens
a hydrogen atom is replaced by an electrophile
An energy profile of the electrophilic substitution of benzene
increase in energy when the delocalised pi (π) system becomes disrupted; also note the decrease in energy when the π system is reformed in the substituted product. The high activation energy required for the reaction to take place is another testament to the stability of the benzene molecule.
The substitution of an electrophile into a benzene ring. Note that the delocalised pi (π) system is restored.
A pair of electrons (represented by a curly arrow) moves from the π system to create a bond with the electrophile. This results in the formation of an unstable carbocation intermediate. Note that the delocalised pi system has been disrupted as discussed previously. The restoration of the stable delocalised pi system is achieved by the loss of a proton (H+) from the carbocation intermediate.
what does benzene react with to form nitrobenzene
Benzene reacts with a mixture of concentrated nitric acid and sulfuric acid (known as a nitrating mixture) to form nitrobenzene, C6H5NO2.
nitration of benzene equation
2H2SO4 + HNO3 ⇌ NO2+ + 2HSO4− + H3O+
mechanism for the nitration of benzene
conditions for the nitration of benezene
The conditions for this reaction are heat under reflux to 50°C; note that the temperature should not be raised higher than 50°C as further nitration to dinitrobenzene will occur.
carbonyl group
C=O
how to reduce an aldehyde to a primary alcohol again
the reduction of an aldehyde, propanal, to a primary alcohol, propan-1-ol. The reducing agent is sodium borohydride (lithium aluminium hydride can also be used) followed by the addition of an acidic solution (H+).
how to reduce a ketone to a secondary alochol again
the reduction of a ketone, propanone, to a secondary alcohol, propan-2-ol. The reducing agent is sodium borohydride followed by the addition of an acidic solution. As with the reduction of aldehydes, it is also possible to use lithium aluminium hydride as the reducing agent.
how to reduce a carboxylic acid to a primary alcohol again
The reduction of carboxylic acids requires the use of the stronger reducing agent, lithium aluminium hydride (LiAlH4), in dry ether.
can you stop the reactioon of carboxylic acid and lithium aluminum hydride when its an aldehyde
no it reacts too quickly
how is the reduction of nitrobenzene
two stage reduction reaction
Firstly, nitrobenzene, C6H5NO2, is heated under reflux (using a boiling water bath) together with a mixture of tin (Sn) and concentrated hydrochloric acid. The product, the phenylammonium ion, C6H5NH3+, is protonated as the reaction is carried out under strongly acidic conditions. In the second stage, the phenylammonium ion is deprotonated to produce phenylamine by reacting with sodium hydroxide solution.
In the first stage, nitrobenzene is reacted with tin in an acidic solution to form tin(II) ions, the phenylammonium ion and water. The equation for this reaction is:
C6H5NO2 (l) + 3Sn (s) + 7H+ (aq) → C6H5NH3+ (aq) + 3Sn2+ (aq) + 2H2O (l)
In the second stage, the phenylammonium ion is reacted with hydroxide ions to form phenylamine (aniline) and water:
C6H5NH3+ (aq) + OH– (aq) → C6H5NH2 (l) + H2O (l)
what is retrosynthesis
One method of devising organic compounds involves starting with the desired product (the target molecule) and working backwards – a technique known as retrosynthesis. By looking at the functional groups on the target molecule, chemists can identify the preceding steps in the synthesis, starting with readily available materials. This can be summarised as shown below:
target molecule ⇒ precursors ⇒ starting materials
what is stereoisomerism
Stereoisomers have the same structural formula but differ in the spatial arrangement of atoms
how can stereoisomers be divided
configurational isomers and conformational isomers
config isomers
cannot be converted by rotations about single bonds
conformational isomers
are compounds that can be converted just by rotations about single bonds
optical isomers
are mirror images showing chirality
newman projections
used to represent conformers (conformational isomers)
The difference in energy between the two conformers
torsional strain
According to the Cahn–Ingold–Prelog (CIP) priority rules
the atom with the highest atomic number on each carbon atom in the double bond is assigned the highest priority.
if both high priority groups are on the same side…
z isomer
if both high priority groups are on different sides
e isomer
in which molecules does optical isomerism occur in
chiral molecules with a chiral centre
what is an asymmetric/chiral carbon atom
one that is bonded to four different atoms or groups
enantiomers
two stereoisomers that are mirror images of eachother
how many optical isomers are there for an organic molecule with n chiral centres
2^n
what is non sueprimposable
it is not possible to superimpose one enantiomer onto another
two enantiomers are said to be optically active, meaning
they rotate the plane of plane polarised light
Plane-polarised light is produced when
unpolarised light is passed through a polarising filter (Figure 3). If a vertical polarising filter is used, then only the vertical plane passes through. The plane of light that passes through the filter is known as plane-polarised light.
do the two enantiomers have identical chemical and physical properties
yes apart from their interaction with plane polarised light
how to distinguish between enantiomers
When plane-polarised light is passed through a solution of each enantiomer, they rotate the plane of the plane-polarised light in equal but opposite directions.
what composes a polarimeter
The rotation of the plane of plane-polarised light by the two different enantiomers can be measured using a polarimeter (Figure 4). A polarimeter comprises a light source, two polarising filters (one fixed and one that can be rotated) and a tube that contains a solution of the enantiomer.
operation of a polarimeter
Unpolarised light is passed through a polarising filter, which produces plane-polarised light.
The plane-polarised light passes through a solution of the enantiomer.
An analyser is used to determine the angle of rotation of the plane of the plane-polarised light.
When both enantiomers are present in the solution in equal amounts, the two opposite rotations of the plane of the plane-polarised light will cancel each other out. Such a mixture is said to be
optically inactive and is referred to as a racemic mixture
what are disastereoisomers or diastereoisomers
Some molecules do not have opposite configurations at all of the chiral centres and so are not mirror images of each other.
can diastereoisomers be optically active
yes but some are not
which reagents are needed to convert nitrobenzene to phenylamine in 2 steps?
- tin and conc HCl
- sodium hydroxide
