Organic Chemistry Flashcards
4 steps to name a compound
- Identify the longest carbon chain (meth-, eth-, prop-, but-)
- Identify the type of bonding (sing. -an-, dbl. -en-, trp. -yn-)
- Identify the functional group (hydrogen -e, hydroxyl -ol, amino- , chloro- …)
- Numbers are used to give the position of groups or bonds along the chain.
Name all 7 functional group suffix/prefix (not halogen)
Hydrogen (-e)
Hydroxyl (-ol)
Amino (amino-)
Aldehyde (-al)
Ketone (-one)
Carboxyl (-oic acid)
Ester (-oate)
What is an alkyl radical?
If one = hydrogen atom is removed from an alkane what is left is known as an alkyl radical.
eg. ethyl C2H5
What is a Homologous series?
Homologous series have the same
general formula with the neighbouring
members of the series differing by
-CH2-
The chemical properties of the individual members of a homologous series
are similar and they show a gradual change in physical properties.
Ester
Functional group
Suffix
Example
General formula
Ester - (-COOR)
-Anoate
-Methyl propanoate
CnH2n+1COOH
Alcohol
Functional group
Suffix
Example
General formula
-OH
-anol
Ethanol
CnH2n+1OH
Aldehyde
Functional group
Suffix
Example
General formula
R-O-R’
-anal
Propanal
R-CHO
Ketone
Functional group
Suffix
Example
General formula
RCR’O
-anona
propanone
R-CO-R’
Carboxylic acid
Functional group
Suffix
Example
General formula
COOH
-anoic acid
propanoic aciid
CnH2n+1COOH
Amide
Functional group
Suffix
Example
General formula
-NH2
-anamine
ethanamine
—
Nitrile
Functional group
Suffix
Example
General formula
CN
-anenitrile
propaneniitrile
—
Arene
Functional group
Suffix
Example
General formula
C6H5
-benzene
methyl benzene
—
Classification of Alcohols and halogenoalkanes
primary - one R group
Secondary - two R groups
tertiary - Three R groups
What is the consequence of branching on boling point?
When branching occurs the molecules
become more spherical in shape, which
reduces the contact surface area between
them and lowers the boiling point.
What affects solubillity?
increased length reduces solubulity
What is a subsitution reaction
Alkanes can react with chlorine (or other halogens) in the presence o ultraviolet light to orm hydrogen chloride and a substituted alkane, e.g. methane can react with chlorine to orm chloromethane and ethane can react with bromine to orm bromoethane.
What ia the difference in heterolytic fission and homolytic fission
In heterolytic fssion both o the shared electrons go to one o the atoms resulting in a negative and a positive ion. In homolytic fssion each o the two atoms orming the bond retains one o the shared electrons resulting in the ormation o two free radicals
MECHANISM OF CHLORINATION OF METHANE
The bond between two halogen atoms is weaker than
the CH or CC bond in methane and can break homolytically in the presence o ultraviolet light.
This stage o the mechanism is called initiation.
Free radicals contain an unpaired electron and are highly reactive. When the chlorine ree radicals come into contact with a methane molecule they combine with a hydrogen atom to produce hydrogen chloride and a methyl radical.
Since a new radical is produced this stage o the mechanism is called propagation. The methyl ree radical is also extremely reactive and reacts with a chlorine molecule to orm the product and regenerate another chlorine radical. This is a urther propagation step and enables a chain reaction to occur as the process can repeat itsel.
In theory a single chlorine radical may cause up to 10 000 molecules o chloromethane to be ormed. Termination occurs when two radicals react together.
What are the reactants in addition reactions?
Always an Alkene +
Water (conc H2SO4 cat.) or
Bromine or
Hydrogen or
Hydrogen halide
Alkene + Hydrogen produces
An alkane
Alkene + Water produces (Hydration)
An alcohol if conc. H2SO4 is used as a catalyst
Alkene + hyddrogen Halide
Halogenoalkane
Alkene + Halogen
Dihalogenoalkane
Describe the oxidation of ethanol
Ethanol can be readily oxidized by warming with an acidied solution o potassium dichromate(VI). During the process the orange
dichromate(VI) ion Cr O 2 - is reduced rom an oxidation state o +6 to the green Cr3 + ion
The product of this is ethanal which is very volitle due its lack of hydrogen bonding. If the aldehyde is sufficent than the ethanal can be distilled.
If further oxidation is required than the reactioon should continue in a reflux where the ethanal is oxidised to ethanoic acid.`
How are primary alcohols oxidised
All primary alcohols are oxidized by acidied potassium dichromate(VI), rst to aldehydes then to carboxylic acids.
How are secondary alcohols oxidised?
Secondary alcohols are oxidized to ketones, which cannot undergo urther oxidation.
Why do tertiary alcohols not oxidise readily?
Tertiary alcohols cannot be oxidized by acidied dichromate(VI) ions as they have no hydrogen atoms attached directly to the carbon atom containing the -OH group. It is not true to say that tertiary alcohols can never be oxidized, as they burn readily, but when this happens the carbon chain is destroyed.
How do subsitution reactions of halgenoalaknes occur?
Because of the greater electronegativity of the halogen atom compared with the carbon atom halogenoalkanes have a polar bond. Reagents that have a non-bonding pair o electrons are attracted to the electron-decient carbon atom in halogenoalkanes and a substitution reaction occurs. Such reagents are called nucleophile
The halogen breaks off as a negatve ion
Why does benzene not undergo addition reactions?
The extra stability provided by the delocalization o the electrons in the benzene ring means that benzene, unlike simple alkenes, does not readily undergo addition reactions
Why does benzene undergo subsitution reactions and describe two examples.
Benzene has a high electron density so reacts with electrophiles
Benzene + chlorine in the presence o aluminium chloride (the electrophile is Cl+ ) to orm chlorobenzene
Benzene + nitric acid in the presence
o suluric acid and heat (the electrophile is NO + ) to orm nitrobenzene and water.
What is the process of esterification?
Alcohols react with carboxylic acids in the presence o a small amount o concentrated suluric acid, which acts as a catalyst, to orm an ester and water
What is a condensation reaction?
A condensation reaction involves the reaction between two molecules to produce a larger molecule with the elimination o a small molecule such as water or hydrogen chloride.
How do biofuels form from fats?
In a process known as transesterication fats can react with alcohols in the presence o sodium hydroxide (which acts as a catalyst) to orm alkyl esters, which can be used as biouel and glycerol
What solvent is used for Sn1 reactions?
Protic solvents which are polar, such as water or ethanol, avour
the SN1 mechanism as they support the breakdown
of halogenoalkanes into carbocations and halide ions
What solvents are used for a Sn2 reaction?
Aprotic solvents which are less polar such as ethoxyethane, favour an S 2 mechanism involving a transition state
Which factors affect the rate of nucleophilic reactions?
The nature ofthe nucleophile
The eectiveness o a nucleophile depends on its electron density. Anions tend to be more reactive than the corresponding neutral species.
The nature of the halogen
Iodoalkanes react aster than bromoalkanes, which in turn react aster than chloroalkanes. This is due to the relative bond enthalpies as the CI bond is much weaker than the CCl bond and thereore breaks more readily.
The nature ofthe halogenoalakane
Tertiary halogenoalkanes react aster than secondary halogenoalkanes which in turn react aster than primary halogenoalkanes. The SN1 route
which involves the ormation o an intermediate carbocation is aster than the S 2 route which involves a transition N state with a relatively high activation energy.
Describe the mechnaism. for the addition of ethene and Hydrgen Bromide
The reaction can occur in the dark which suggests that a ree radical mechanism is not involved. The double bond in the ethene molecule has a region o high electron density above and below the plane o the molecule. Hydrogen bromide
is a polar molecule due to the greater electronegativity o bromine compared with hydrogen. The hydrogen atom (which contains a charge o +) rom the HBr is attracted
to the double bond and the HBr bond breaks, orming a bromide ion. At the same time the hydrogen atom adds to one o the ethene carbon atoms leaving the other carbon atom with a positive charge. A carbon atom with a positive charge is known as a carbocation. The carbocation then combines with the bromide ion to orm bromoethane. Because the hydrogen bromide molecule is attracted to a region o electron density it is described as an electrophile and the mechanism is described as electrophilic addition.
Why does bromine react with ethene like hydrogen bromide does to complete an addition reaction?
Electrophilic addition also takes place when bromine adds to ethene in a non-polar solvent to give 1,2-dibromoethane. Bromine itsel is non-polar but as it approaches the double bond o the ethene an induced dipole is ormed by the electron cloud.
What is an asymetric alkene?
Asymmetric alkenes contain different groups attached to the carbon atoms o the C=C bond.
What is MARkOVNIkOVfS RULE?
MARkOVNIkOVfS RULE allows us to predict the. major products of. the reaction between an asymmetrical alkene and a hydrogen halide.
The rule states that the Hydrogen from the hydrogen halide will bind with the carbon atom ( from the carbon atoms with double bonds) with most hydrogens bonded. The halogen will bind to the other carbon atom which was oriiginally in the carbon bond.
Is Sn1 or Sn2 applied in primary, secondary and tertiary halogenoalkanes?
For tertiary halogenoalkanes the predominant mechanism is SN1 and for primary halogenoalkanes it is SN2. Both mechanisms occur for secondary halogenoalkanes.
What determines the rate of Sn1 and Sn2 reactions?
The rate determining step (slow step) in an SN1 reaction depends only on the concentration of the halogenoalkane, rate = k[halogenoalkane]. For SN2, rate = k[halogenoalkane][nucleophile].
Describe the bonds in benzene
Benzene is the simplest aromatic hydrocarbon compound (or arene) and has a delocalized structure of π bonds around its ring. Each carbon to carbon bond has a bond order of 1.5. Benzene is susceptible to attack by electrophiles.
Explain MARkOVNIkOVfS RULE
When hydrogen ions react with propene two dierent carbocation intermediates can be ormed.
The first one has the general formula RCH+ and is a primary carbocation
The second one has two R- groups attached to the positive carbon ion R2CH+ and is known as a secondary carbocation
A tertiary carbocation has the general ormula R3CH+ .
The R-groups (alkyl groups) tend to push electrons towards the carbon atom they are attached to which tends to stabilize the positive charge on the carbocation. This is known as a positive inductive effect. This effect will be greatest with tertiary carbocations and smallest with primary
Applying MARkOVNIkOVfS RULE predict the major product when Iodine chloride (ICl) reacts with but-1-ene
Consider the reaction o iodine chloride ICl with but-1-ene. Since iodine is less electronegative than chlorine the iodine atom will act as the electrophile and add frst to the alkene.
The major product will thus be 2-chloro-1-iodobutane
(page 91 of revision book)
Describe the nitration of benzene
Benzene reacts with a mixture o concentrated nitric acid and concentrated suluric acid when warmed at 50 C to give nitrobenzene and water. Note that the temperature should not be raised above 50 C otherwise urther nitration to dinitrobenzene will occur.
The electrophile is the nitryl cation NO2 +
nitronium ion). The concentrated suluric acid acts as a catalyst. Its unction is to protonate the nitric acid which then loses water to orm the electrophile. In this reaction nitric acid is acting as a base in the presence o the more acidic suluric acid.
The NO2+ is attracted to the delocalized pi bond and attaches
to one o the carbon atoms. This requires considerable activation energy as the delocalized pi bond is partially broken. The positive charge is distributed over the remains o the pi bond in the intermediate. The intermediate then loses a proton and energy is evolved as the delocalized pi bond is reormed. The proton can recombine with the hydrogensulate ion to regenerate the catalyst
Which catalyst do we use in the reduction of carbonyl compunds and why?
Sodium tetrahydridoborate NaBH4 can be used in the presence o protic solvents such as water or ethanol but is ineffectual at reducing carboxylic acids. The stronger reducing agent lithium aluminium hydride LiAlH4 must initially be used in aprotic solvents such as ether as it reacts with water, then the reaction is acidied to obtain the product. Another reducing agent that can be used is hydrogen itsel in the presence o a nickel, platinum or palladium catalyst.
How are aldeyhdes reduced?
Aldehydes are reduced to primary alcohols, with NaBH4 or H+ as a catalyst
How are ketones reduced?
Ketones are reduced to secondary alcohols, with NaBH4 or H+ as a catalyst
How are ccarboxylic acids reduced?
Carboxylic acids are reduced to primary alcohols, with LiAlH4 in ethers or H+ as catalyst
How is nitrobenzene reduced?
The reduction of nitrobenzene to phenylamine is usually carried out in two stages.
Stage 1. Nitrobenzene is refuxed with a mixture o tin and concentrated hydrochloric acid. The tin provides the electrons by acting as the reducing agent and the product is the phenylammonium ion.
Stage 2. The addition o sodium hydroxide solution releases the free amine
Why are the shortest synthesis routes prefered?
The more steps there are in an organic synthesis, then the lower the
nal yield is likely to be, because some material will be lost during each step.
What are isomers?
Isomers are compounds that are composed o the same elements in the same proportions but dier in properties because o dierences in the arrangement o atoms
What is a structural isomer?
Structural isomers share the same molecular ormula but have dierent structural ormulas.
What are stereoisomerism?
Stereoisomers have the same structural ormula but dier in their spatial arrangement
What is a CIS and TRANS isomer?
A cis-isomer is one in which the substituents are on the same side o the double bond. In a trans-isomer the substituents are on opposite sides o the double bond.
When does CIS-Trans and E/Z isomerism occur?
Both cistrans and E/Z isomerism occur when rotation about a bond is restricted or prevented.
This is because the double bond in an alkene is made up of a sigma and a pi bond. The pi bond is formed from the combination of two p orbitals, one from each of the carbon atoms. These two p orbitals must be in the same plane to combine. Rotating the bond would cause the pi bond to break so no rotation is possible.
Cistrans isomerism can also occur in disubstituted cycloalkanes. The rotation is restricted because the CC single bond is part o a ring system.
What is E/Z iisomerism?
E/Z terminology is quite easy to apply and depends on what are known as the Cahn-Ingold-Prelog (CIP) rules for determining the priority of the atoms or groups attached to the two carbon atoms of the double bond. In simple terms the higher the atomic number of the attached atoms
to each carbon atom the higher the priority.
If. both entities of highest priotiry lie on the same side we haze Z-isomerism, if both of highest priority are on on oppisite sides. there is E isomerism.
(Page 95 in revision book)
When does optical isomerism occur?
Optical isomerism is shown by all compounds that contain at least one asymmetric or chiral carbon atom within the molecule, that is, one that contains our dierent atoms or groups bonded to it, also known as a stereocentre. The two isomers are known as enantiomers and are mirror images o each other
How are optical isomers distoguished?
The two dierent isomers are optically active with plane-polarized light. Normal light consists o electromagnetic radiation which vibrates in all planes. When it is passed through a polarizing flter the waves only vibrate in one plane and the light is said to be plane-polarized.
The two enantiomers both rotate the plane o plane-polarized light. One o the enantiomers rotates it to the let and the other rotates it by the same amount to the right.
What is Diastereomerism?
I a molecule contains two or more stereocentres then several dierent stereoisomers are possible. They are known as enantiomers i they are mirror images and as diastereomers i they are not mirror images o each other. Diastereomerism occurs when two or more stereoisomers o a compound have dierent confgurations at one or more (but not all) o the equivalent stereocentres. This is particularly important with many sugars and some amino acids. Diastereomers have dierent physical properties and dierent chemical reactivity.
How is optical activity measured?
The optical activity o enantiomers can be detected and measured by an instrument called a polarimeter. It consists o a light source, two polarizing lenses, and between the lenses a tube to hold the sample o the enantiomer dissolved in a suitable solvent.
When light passes through the frst polarizing lens (polarizer) it becomes plane-polarized. That is, it is vibrating in a single plane. When the sample is placed between the lenses the analyser must be rotated by X degrees, either clockwise (dextrorotatory) or anticlockwise (laevorotatory) to give light o maximum intensity. The two enantiomers rotate the plane o plane-polarized light by the same amount but in opposite directions. If both enantiomers are present in equal amounts the two rotations cancel each other out and the mixture appears to be optically inactive. Such a mixture is known as a racemic mixture or racemate.
How and why does boiling point change in a homologous series?
Boiling point increases with increasing carbon number. This is because of the increased instantaneous induced dipoles causing stronger London (dispersion) forces between the molecules as their molecular size increases
What is a primary carbon atom?
A primary carbon atom is attached to the functional group and also to at least two hydrogen atoms. Molecules with this arrangement are known as primary molecules.
For example, ethanol, C2H5OH, is a primary alcohol and chloroethane, C2H5Cl, is a primary halogenoalkane.
What is a secondary carbon atom?
A secondary carbon atom is attached to the functional group and also to one hydrogen atom and two alkyl groups. These molecules are known as secondary molecules. For example, propan-2-ol, CH3CH(OH)CH3, is a secondary alcohol, and 2-chloroethane, CH3CHClCH3, is a secondary halogenoalkane.
What is a tertiary carbon atom?
A tertiary carbon atom is attached to the functional group and is also bonded to three alkyl groups and so has no hydrogen atoms. These molecules are known as tertiary molecules. For example, 2-methylpropan-2-ol
When is a nitrogen atom in an amine primary, secondary or tertiary?
If the N is attached to one Alkyl group then it is primary, two secondary and three tertiary
Explain bond lengths in benzene
All carbon–carbon bond lengths in benzene are equal and intermediate in length between single and double bonds as each bond contains a share of three electrons between the bonded atoms.
Explain why benzene is reluctant to undergo addition reactions and is more likely to undergo substitution reactions.
Addition reactions are energetically not favoured
as they would involve disrupting the entire cloud of delocalized electrons. The resonance energy would have to be supplied and the product, without the delocalized ring of electrons, would be less stable. Benzene can instead undergo substitution reactions that preserve the stable ring structure
Explain why benzene is reluctant to undergo addition reactions and is more likely to undergo substitution reactions.
Addition reactions are energetically not favoured
as they would involve disrupting the entire cloud of delocalized electrons. The resonance energy would have to be supplied and the product, without the delocalized ring of electrons, would be less stable. Benzene can instead undergo substitution reactions that preserve the stable ring structure
What is resonance energy or stabilization energy of benzene
The amount of energy that would have to be supplied in order to overcome the special stability of the delocalised ring
Why do branched isomers have lower bp?
The intermolecular forces decrease when branching occurs as the surface area is reduced
Why are alkanes generally unreactive?
Alkanes contain only C –– C and C –– H bonds, which are both strong bonds: C –– C = 348 kJ mol–1 and C –– H = 412 kJ mol–1. So these molecules will only react in the presence of a strong source of energy, strong enough to break these bonds. As a result, alkanes are stable under most conditions.
The C –– C and C –– H bonds are also characteristically non-polar, so these molecules are not susceptible to attack by most common reactants.
These two factors taken together mean that alkanes are generally of very low reactivity.
Why do some molecules produce more smoke than others during combustion?
As the C : H ratio increases with unsaturation, there is an increase in the smokiness of the flame, due to unburned carbon.
Methane, CH4, reacts with chlorine producing chloromethane and hydrogen chloride. Explain why this reaction doesn’t occur in the dark?
CH4(g) + Cl2(g) ⎯⎯⎯> CH3Cl(g) + HCl(g)
The reaction does not take place in the dark as the energy of UV light is necessary to break the covalent bond in the chlorine molecule. This splits it into chlorine atoms, which each have an unpaired electron and are known as free radicals. It is known as photochemical homolytic fission. Once formed, these radicals will start a chain reaction in which a mixture of products including the halogenoalkane is formed.
Briefly describe the reaction mechanism for the reaction between Chlorine and an Alkane
First initiation occurs in which the diatomic Chlorine molecule is broken into two chlorine free radicals in the presence of UV light. This is known as photochemical homolytic fission.
Next propagation occurs. Propagation reactions both use and produce free radicals. For example:
Cl+CH4 →CH3+HCl
CH3* + Cl2 → CH3Cl + Cl*
There are many possible propagation steps, which all allow the reaction to continue. This is why this type of reaction is often called a chain reaction.
Last step is termination. Termination reactions remove free radicals from the mixture by causing them to react together and pair up their electrons. For example:
Cl+Cl→Cl2
CH3* + Cl→ CH3Cl
CH3 + CH3* → C2H6
What is hydrogenation and what conditions does the reaction require?
Hydrogen reacts with alkenes to form alkanes in the presence of a nickel catalyst at about 150 °C.
Describe the addition reaction of alkene and bromide
Halogens react with alkenes to produce dihalogeno compounds. These reactions happen quickly at room temperature, and are accompanied by the loss of colour of the reacting halogen.
Describe the addition reaction of alkene and a hydrogen halide
Hydrogen halides, such as HCl and HBr, react with alkenes to produce halogenoalkanes. These reactions take place rapidly in solution at room temperature
All the hydrogen halides are able to react in this way, but the reactivity is in the order HI > HBr > HCl due to the decreasing strength of the hydrogen halide bond down Group 17. So HI, with the weakest bond, reacts the most readily.
Why are alcohols soluble ?
The –– OH group is polar, and so increases the solubility in water of the molecules, relative to alkanes of comparable molar mass?
Why are esters insoluble?
Unlike their parent acid and alcohol, esters have no free –– OH groups so they cannot form hydrogen bonds and so are mostly quite insoluble and form a layer on the surface.
Explain why addition reactions of benzene are possible
Addition reactions, which would lead to loss of the stable arene
ring, are generally not favoured as the products would be of higher energy than the reactants. Instead, substitution reactions, in which one or more of the hydrogen atoms is replaced by an incoming group, occur more readily as these lead to products in which the arene ring is conserved.
Is water or OH- a stronger nucleophile and why?
Because H2O lacks a negative charge it is a weaker nucleophile than OH–.
give the rate equation for SN2 reactions
rate = k [halogenoalkane] [nucleophile]
Why does inversion occur in SN2 reactions?
The nucleophile attacks the electrophilic carbon atom on the opposite side from the leaving group, which causes an inversion of the arrangement of the atoms around the carbon atom
Why are SN2 reactions deemed stereo specific?
The SN2 mechanism is described as stereospecific because the three-dimensional arrangement of the reactants determines the three-dimensional configuration of the products. This happens because bond formation comes before bond cleavage in the transition state, so the stereochemistry of the carbon attacked is not lost.
What are aprotic solvents and why are they used for SN2 reactions?
Aprotic solvents are those which are not able to form hydrogen bonds as they do not contain –OH or –NH bonds, although they may have strong dipoles. This means they solvate the metal cations
(Na+ ) rather than the nucleophile (OH ). The unsolvated, bare nucleophile has a higher energy state and this increases the reaction rate.
Why and how do tertiary compounds undergo SN1 reactions?
The alkyl groups around the carbon of the carbon–halogen bond causes what is called steric hindrance, meaning that these bulky groups make it difficult for an incoming group to attack this carbon atom. Instead, the first step of the reaction involves the halogenoalkane ionizing by breaking its carbon–halogen bond heterolytically. As the halide ion is detached, this leaves the carbon atom with a temporary positive charge, which is known as a carbocation intermediate. This is then attacked by the nucleophile in the second step of the reaction, leading to the formation of a new bond.
Another reason which favours this mechanism in tertiary halogenoalkanes is that the carbocation is stabilized by the presence of the three alkyl groups, as each of these has an electron-donating or positive inductive effect. This stabilizing effect helps the carbocation to persist for long enough for the second step to occur.
Give the rate equation for SN1 reactions
rate = k [halogenoalkane]
Are SN1 reactions stereo specific? Explain you answer.
The carbocation intermediate has a planar shape, which means the nucleophile can attack from any position in the second step. This means that the S stereospecific.
What are protic solvents and why are they used for SN1 reactions?
Polar, protic solvents contain – OH or – NH and so are
able to form hydrogen bonds. They are effective in stabilizing the positively charged intermediate by solvation involving ion–dipole interactions
What is the influence of the leaving group on the rate SN1 and SN2 reactions
As the electronegativity of the halogens decreases in going down the group from fluorine to iodine, the carbon of the carbon–halogen bond becomes progressively less electron deficient and so less vulnerable to nucleophilic attack. So from this we would expect:
fluoroalkane > chloroalkane > bromoalkane > iodoalkane
Bond energy data show that the carbon– halogen bond decreases in strength from fluorine to iodine. As the substitution reaction involves breaking this bond, we would expect the ease of breaking bonds to be:
C –– I > C –– Br > C –– Cl > C –– F
Reaction rate data indicate that (b) above, the strength of the carbon–halogen bond, dominates the outcome here. The relative rate of reaction of the different halogens in halogenoalkanes when all other variables are kept constant is therefore:
iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes
How do alkenes undergo electrophilic addition reactions?
-The carbon atoms of the double bond are sp2 hybridized, forming a planar triangular shape, with bond angle 120°. This is a fairly open structure that makes it relatively easy for incoming groups to attack.
* The pi (π) bond represents an area of electron density above and below the plane of the bond axis. Because electrons in the π bond are less closely associated with the nuclei, it is a weaker bond than the σ bond and so breaks more easily during the addition reactions.
* Because it is an area of electron density, the π bond is attractive to electrophiles, species that either are electron deficient or that become electron deficient in the presence of the π bond.
When this bond breaks, reactants attach at each carbon atom:
Outline how bromine reacts with ethene with reference to electrophilic addition
Bromine is a non-polar molecule, but as it approaches the electron rich region of the alkene, it becomes polarized by electron repulsion.
The bromine molecule is polarized by an alkene
The bromine atom nearest the alkene’s double bond gains a δ+ charge and acts as the electrophile. The bromine molecule splits heterolytically, forming Br+ and Br−, and the initial attack on the ethene in which the π bond breaks is carried out by the positive ion, Br+
This step is slow, resulting in an unstable carbocation intermediate in which the carbon atom has a share in only six outer electrons and carries an overall positive charge.
This unstable species then reacts rapidly with the negative bromide ion, Br−, forming the product 1,2-dibromoethane.
For hydrogen halides the same process occurs with the H+ making the first attack after heterolytic fission!
Outline electrophilic substitution
The reaction has high activation energy and so proceeds rather slowly. This is because the first step in the mechanism, in which an electron pair from benzene is attracted to the electrophile, leads to a disruption of the symmetry of the delocalized π system. The unstable carbocation intermediate that forms has both the entering atom or group and the leaving hydrogen temporarily bonded to the ring. Loss of the hydrogen ion, H+, from this intermediate leads to the electrically neutral substitution product as two electrons from the C–H bond move to regenerate the aromatic ring.
What is thiol oxidation?
The oxidation of thiols gives compounds called disulfides. Two thiol molecules are required for the oxidation to form the disulfide.
