Electron sharing and pair-sharing reactions

radical

• a chemical species that has an unpaired electron

• can be described as:

  • atomic: single atom

  • polyatomic: group of atoms bonded together with no overall charge

  • anionic: atom/molecule that gains an electron to become anion and has an atom with an unpaired electron

  • cationic: atom/molecule that loses an electron to become cation and has an atom with an unpaired electron

reactivity of radicals

• unpaired electron makes them highly reactive

• high enthalpy

• it is energetically favourable for radicals to react and form products with a lower enthalpy, which can be done by:

  • taking an electron from another species

  • combining with another radical to form a covalent bond

homolytic fission

• homolytic fission is breaking a covalent bond in a way that each atom takes an electron from the bond to form 2 radicals

• movement of a pair of electrons is shown using a curly arrow, and a fish hook for one electron

• since bond breaking is endothermic, energy is required for homolytic fission

• amount of energy needed depends on bonds present:

  • for weaker bonds, heat is sufficient (thermolytic fission)

  • for stronger bonds, UV light is necessary (photolytic fission)

halogenation of alkanes

• alkanes are very stable and non-polar due to the near identical electronegativities of carbon and hydrogen, so they are very difficult to break

• they can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen

• UV light is necessary as alkanes are very unreactive

• mechanism

  • initiation: homolytic fission of halogen

  • propagation: radicals create further radicals

  • termination: 2 radicals collide with eachother

propagation step

• reactive radicals will attack the unreactive alkanes

• a C-H bond breaks homolytically, forming an alkyl free radical

• the alkyl free radical is also extremely reactive and can react with a halogen molecule to regenerate another halogen radical, the process can repeat itself

termination step

• when 2 radicals react together to form a non-radical product

• multiple products are possible

nucleophile Nu:-

• electron-rich species that can donate a pair of electrons to form a coordinate bond

• all have LP, can be negative or neutral

• nucleophilic reaction: nucleophile attacks carbon atom which carries partial positive charge

• nucleophile replaces the atom with a partial negative charge

nucleophilic substitution

• halogenoalkanes undergo nucleophilic substitution due to the polar C-X bond

• nucleophiles form a coordinate bond to the carbon

• the bond between carbon and halogen leaves, halogen leaves as the halide ion (leaving group)

• the lower the halogen in the group, the faster the rate of reaction/substitution due to decreasing bond enthalpy

heterolytic fission

• breaking a covalent bond so that the more electronegative atom takes both the electrons from the bond, forming a negative ion and leaving behind a positive ion

• the negative ion is a nucleophile, the positive ion is an electrophile

electrophile E+

• species that forms a covalent bond when reacting with a nucleophile by accepting electrons

• electron deficient, so will have positive or partial positive charge

• e.g H2O, halogens

electrophilic addition

• addition of an electrophile to an alkene double bond, an area of high electron density

  • contain pi bonds which are easier to break, meaning alkenes can undergo addition reactions

  • this makes alkenes much more reactive than alkanes

• C=C bond breaks by forming a single bond and 2 new bonds from each C atom

addition of water (hydration)

• alkenes treated with steam at 300^oC , 60 atm pressure and sulphuric acid catalyst

• water is added across double bond, converting alkene into alcohol

• C₂H₄ —→ C₂H₅OH

• water is weak electrophile so doesn’t undergo addition reactions unless in presence of strong acid catalyst

• step 1

  • pi electrons in C=C are attracted to H3O+

  • heterolytic fission occurs forming carbocation

• step 2

  • water acts as a nucleophile and donates a pair of electrons to the positive carbon atom forming a C-O bond

  • equilibrium established between positive product and alcohol

addition of halogens (halogenation)

• electrophile (halogen) joins onto double bond, results in dihalogenoalkane

• halogens can be used to test whether a molecule is unsaturated

  • unknown compound shaken with bromine water which is yellow, if unsaturated, addition reaction will occur and coloured solution will decolourise

• halogens have a temporary dipole caused by repulsion of halogens by the high electron density of the C=C bond

• follow same mechanism as in hydrogen halides

addition of hydrogen halides (hydrohalogenation)

• hydrogen and halogen added across double bond

• due to decreasing bond enthalpy down the group, the fastest reaction is HI

• hydrogen halides are polar due to different electronegativities

  • halogen has a partial negative charge, and hydrogen partial positive

• the H atom acts as electrophile and lewis acid by accepting a pair of electron from C=C in alkene

  • H-X breaks heterolytically forming X- ion

• this forms a highly reactive carbocation which reacted with X- ion

polymers

• large molecules made from repeating subunits (monomers)

• natural polymers: proteins, DNA

• synthetic polymers: plastics

addition polymers

• many monomers containing at least one C=C double bond form long chains of polymers as the only product

properties of plastics

• low weight

  • polymers loosely packed so less dense and lighter

• unreactive

  • addition polymers from alkenes are saturated and non polar so unreactive

• water resistant

  • polymers are hydrophobic

• strong

  • polymers are made up of many strong covalent bonds

condensation polymerisation

• polymer is produced by repeated condensation reactions between monomers

• involves the elimination of a small molecule (in natural condensation polymerisation this is water)

polyester

• polymer formed by condensation polymerisation of dicarboxylic acid and diol monomers

  • diol: contains 2 OH groups

  • dicarboxylic acid: contains 2 carboxlic acids -COOH

• when polyester is formed, an H atom on one of the diol and an OH group from one of carboxylic acid groups are eliminated as water

• can also be formed using a single monomer, a hydroxycarboxylic acid containing both necessary functional groups

polyamides

• polymers where repeating units are bonded together by amide links

• amide group: -CONH

• diamine and dicarboxylic acid required

  • OH from COOH and H from NH2 expelled as water molecule forming amide link

biodegradable polymers

• polyesters and polyamides can be broken down using hydrolysis, making them better for the environment than addition polymers

lewis acids and bases

• lewis acid: lone pair acceptor

• lewis base: lone pair donor

bronsted-lowry vs lewis

• bronsted-lowry acid: species that can donate H+

• lewis acids cover a broader spectrum, they can accept a lone pair of electrons which includes H+. bronsted-lowry considers acids as H+ donors only

• bronsted-lowry base: species that can accept H+

formation of coordination bonds

• when NH3 and BF3 react, coordinate bond is formed

• occurs as lone pair on nitrogen atom can be donated to boron (electron deficient) creating NH3BF3

• NH3 acts as lewis base (nucleophile) and BF3 as lewis acid (electrophile)

• since only electrons are being donated/accepted, neither is a bronsted-lowry acid/base

lewis acids/bases in complex ions

• complex ion: a central transition mental ion surrounded by ligands bonded to it by coordinate bonds

  • ligands are lewis bases, so must have a lone pair and/or a negative charge

• transition metal acts as the electrophile (lewis acid) and ligands act as nucleophiles (lewis bases)

• different ligands form different numbers of coordination bonds to central metal ion

  • monodentate

  • bidentate

  • polydentate

• coordination number: number of coordinate bonds to central metal

  • can be same as the number of ligands if they are monodentate

representing complex ions

• square brackets used to group together ligands and metal ion

• overall charge is the sum of oxidation states of all species present

• complexes with coordination number of 4 are usually tetrahedral, coordination number 6 usually octahedral

bidentate ligands

• each form 2 co-ordinate bonds to central metal ion

• because each ligand contains 2 atoms with lone pairs

Sn1 reactions

• nucleophilic substitution

• occurs in tertiary halogenoalkanes (carbon bonded to halogen is bonded to 3 alkyl groups)

• 1 = the rate of reaction (determined by slowest step) depends on conc of halogenoalkane

Sn1 mechanism

• 2 step reaction

• 1st step

  • C-X bond breaks heterolytically, halogen leaves as an X^- ion (this is the slow step)

  • forms tertiary carbocation intermediate (tertiary carbon atom with positive charge)

• 2nd step

  • tertiary carbocation is attacked by nucleophile

Sn2 reaction

• nucleophilic substituion

• occurs in primary halogenoalkanes (carbon bonded to halogen bonded to 1 alkyl group)

• 2 means rate is dependent on conc of halogenoalkane and nucleophile ions

Sn2 mechanism

• 1 step

  • nucleophile donates a pair of electrons to partially position C atom forming a new bond

  • at the same time, C-X bond breaks by heterolytic fission and halogen leaves as X^-

• sometimes the halogen atom in halogenoalkane causes steric hinderance

  • the nucleophile can only attack from the opposite side of the C-X bond

  • as a result, the molecule undergoes an inversion of configuration

what influences rates of nucleophilic substitution

• the nature of nucleophile

• the halogen involved

• the structure of the halogenoalkane

the nature of the nucleophile

• the greater the electron density on nucleophile, the stronger the nucleophile

  • negative anions tend to be more reactive than their corresponding neutral species e.g OH^- and water molecules

• when nucleophiles carry the same charge, electronegativity of atom carrying the lone pair becomes deciding factor

  • the less electronegative the atom carrying the lone pair, the stronger the nucleophile

  • this is because a less electronegative atom has a weaker grip on its lone pair

the halogen involved

• substitution reactions break the C-X bond, so bond energies can be used to explain their different reactivities

  • so F>Cl>Br>I in order of most to least strong

the structure of the halogenoalkane

• tertiary halogenoalkanes undergo Sn1, forming stable tertiary carbocations

• secondary halogenoalkanes undergo a mix of both Sn1 and Sn2 reactions depending on structure

• primary halogenoalkanes undergo Sn2 reactions forming less stable primary carbocations

• this is bc of positive inductive effect of the alkyl groups attached to the C atom bonded to the halogen

  • alkyl groups push electron density towards the positively charged carbon reducing the charge density

  • in tertiary carbocations, 3 alkyl groups stabilise the carbocation making them more stable than primary which only have 1

asymmetric alkenes

• contain different groups attached to carbon atoms of the C=C bond

markovnikov’s rule

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

• markovnikov’s rule predicts which isomer will be the major product

  • the H atom will add to the C atom that already contains the most H atoms bonded to it

explanation of markovnikov’s rule

• stability of intermediate carbocation must be considered

• alkyl groups have a positive inductive effect (they push electron density away towards the positive charge density of the carbocation, which partially stabilises the charge)

• when charged carbon surrounded by 1 alkyl group it is primary, 2 second, 3 tertiary

• the more alkyl groups, the more stable the carbocation, the more likely the reaction mechanism goes by that intermediate

electrophilic substitution in benzene

• nitration: substitution of a hydrogen atom from the benzene ring with an electrophile

• benzene combined with nitric acid and catalysed w/sulphuric acid at 50 Celsius

• step 1: generation of electrophile

  • delocalised pi system is extremely stable and a region of high electron density

  • electrophile for nitration is nitronium NO₂+

  • NO₂+ is attracted to delocalised pi electrons

• step 2: electrophilic attack

  • pair of electrons from benzene ring is donated to electrophile and to form covalent bond

  • bond is formed between carbon atom and electrophile forming carbocation intermediate

  • disrupts aromaticity in ring

• step 3: regenerating aromaticity

  • C-H bond breaks and leads to reforming of benzene ring, electrons in bond go to benzene ring

  • products: nitrobenzene and H+

• H+ combined with H₂SO₄^- to reform catalyst