Chemistry - Organic Chemistry: Reactivity

radical

• chemical species that has an unpaired electron (the only requirement)

• indicated by a dot on the chemical species

• if radical is made of several atoms dot is on atom with the unpaired electron

• eg. CH3

reactivity of radicals

• unpaired electron of radical makes it highly reactive with high enthalpy

• favourable for radicals to react with lower enthalpy by: taking an electron from other species (which creates another radical) or combining with another radical to form a covalent bond

• high reactivity means they are typically not very long lasting

homolytic fission

• breaking covalent bonds by each atom taking an electron from the bond to form two radicals

• homolytic fission of halogens is the initiation step in a sequence of steps that form a chain reaction

types of homolytic fission

1. thermolytic (heat)

2. photolytic (UV light)

reactivity of alkanes

• relatively stable/un reactive due to strengths of c-c and c-h bonds

• electro-negativities of carbon and hydrogen bonds are almost the same in alkanes so molecule is non polar

• no electron deficient or electron rich areas to attract electrophiles/nucleophiles

• only react in combustion reactions and undergo substitution by radicals

free radical substitution of alkanes

• atom substituted by halogen

• UV needed due to un-reactive nature of alkanes

• 3 steps: initiation step (halogen broken by UV energy to form two radicals) propagation step (radicals create further radicals in chain reaction) termination step (two free radicals collide)

propagation step

• progression of substitution reaction in a chain reaction

• radicals are very reactive and attack un-reactive alkanes

• C-H bond breaks homolytically

• alkyl free radical produced

• can attack other halogens to form halogeno-alkanes and regenerate halogen radical

termination step

• two free radicals react together, forming single un-reactive molecule

• multiple products possible

nucleophilie

electron-rich species that can donate a pair of electrons

nucleophilic substitution reaction

nucleophile attacks a carbon atom that carries partial positive charge, an atom with partial negative charge is replaced by the nucleophile

hydrolysis of haloalkanes

• haloalkanes undergo nucleophilic substitution due to polar c-x bond

• nucleophile is OH-

• aqueous solution of NaOH or KOH with ethanol is used for OH

• Halogen becomes leaving group

why are halogens good leaving groups

• have relatively weak bonds with carbon

• have high electronegativity so the bonded electrons are drawn to halogen making carbon partially positive and susceptible to nucleophilic attack

• rate of reaction depends in type of halogen in haloalkane

• C-F, C-Cl, C-Br, C-l

neutral nucleophiles

when nucleophile is neutral like water initial product is positive. positive molecule then de-protonates and loss an H+ , becoming neutral.

heterolytic fission

• breaking covalent bond so more electro-negative atom takes both electrons

• forming a electrophile and a nucleophile

• opposite of this is a coordination bond

electrophilic addition

• addition of electrophile to alkene double bond

• area of high electron density so very susceptible to the attack

• bond breaks forming single c-c bonds

• eg. steam to form alcohol

why does the double bon react with electrophiles

since alkene contains pi bonds it is possible to break the weaker pi bonds and create stronger sigma bonds. therefore alkanes can undergo addition reactions and are considered more reactive.

addition of water

alkenes react with steam (300, 60 Atmospheres, sulphuric or phosphoric acid) and water adds across double bond (hydration) so alkene to alcohol

Lewis definitions

Lewis acid: lone pair acceptor (electrophile)

Lewis bace: lone pair donor (nucleophile)

Bronsted-Lowry definitions

Acid: can donate H+

Base: can accept H+

complex ion

central transition metal ion surrounded by ligands which are bonded by dative covalent bonds

different ligands

monodentase, bidentase, multidentase

charge on complex ions

sum of the oxidation states of all the species present, so if ligands are neutral: overall charge is the same as the oxidation state of metal ion

coordination number: number of coordination bonds to metal ion

bidentase ligands

can each form two coordinate bonds to central metal ion because each ligand has 2 atoms with lone pairs

nucleophilic substitution in halogenoalkanes

• halogen replaced by a nucleophile

• can occur in two ways: sn1 and sn2

sn1

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

• 1 because rate of reaction depends on concentration of 1 reactant

• 2 step equation

steps of sn1

1. c-x bond breaks heterolytically and halogen leaves as an x- ion. (slow step)

2. tertiary carbocation is attacked by nucleophile

   so energy profile has two transition states and a carbocation intermediate (exothermic)

sn2

• in primary halogenoalkanes c is bonded to 1 alkyl group

• one step reaction

• nucleophile donates electron to positive carbon to form new bond and at the same time bond breaks and halogen takes both electrons (heterolytic fission)

• remember to draw transition state in mechanism

steric hindrance

• halogen causes steric hindrance

• nucleophile can only attack from opposite side of C-Br

• so there is an inversion of configuration

factors affecting rate of nucleophilic substitution

1. nature of nucleophile

2. halogen

3. structure of halogenoalkane

nature of nucleophile

• more negative charge means higher electron density and stronger nucleophile

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

• lower electronegativity means stronger nucleophile

class of halogenoalkane

tertiary: sn1 (most stable)

secondary: mixed

primary: sn2

electrophilic addition reactions

• addition of electrophile to alkene double bond

• eg. hydrogen, steam, hydrogen halides, halogens

addition of hydrogen halides

• molecule is polar

• h atom acts as an electrophile by accepting electrons from double bond in alkene

• h-br breaks heterolytically

• formation of highly reactive carbocation intermediate which reacts with bromine ion

addition of halogens

• same as halides with one exception

• halide have permanent dipole whereas halogens have temporary dipole induced by repulsion of halogens by high electron density in the double bond

addition of water

• water is a weak electrophile so needs strong acid catalyst (H3O+)

• 2 step reaction

  1. pi electrons in double bond are attracted to catalyst. heterolytic fission and carbocation is formed.

  2. water acts as a nucleophile and donates a pair of electrons to the positive carbon atom. forming c-o. Equilibrium is established between positive product and deprotonated product (alcohol) and catalyst is regenerated

carbocations definition

positively charged carbon atoms with only 3 covalent bonds instead of 4

inductive effect

• alkyl groups attached to positively charged carbon are electron donating groups

• illustrated by arrowheads to show alkyl groups pushing electrons towards positive carbon, decreasing its charge

• so charge is spread more in carbocation making it more stable

• therefore tertiary carbocations are the most stable

Markovnikov’s rule

• predicts outcomes of electrophilic addition

• in an addition of HX to an alkene the halogen ends up bonded to the most substituted carbon atom

• in an addition of interhalogen to alkene the most electronegative halogen ends up bounded to the most substituted carbon.

• applies to unsymmetrical alkenes

• Markovnikov addition favours formation of major product

mechanism of addition in unsymmetrical alkene

2 ways for electrophile to attach

1. break double bond and attach to least substituted carbon (the one bonded to the most other carbons) creating the most stable carbocation and thereby forming major product

2. break double bond and attach to most substituted carbon, creating least stable carbocation and forming minor product

reactions in benzene

• benzene undergoes a wide range of reactions including combustion and nitration

• nitration involves substitution of a hydrogen atom from the benzene ring with an electrophilic atom/group of atoms

steps of electrophilic substition

1. generation of an electrophile

2. electrophile attack

3. regenerating aromaticity

generation of an electrophile

• delocalised pi system is stable and there is an increase in electron density

• so first step is generating electrophile

• in nitration that is nitronium ion no2+ produced in situ by adding concentration nitric and sulfuric acid to between 25-60 degrees into the mixture

electrophile attack

• pair of electrons from benzene are donated to electrophile as covalent bond

• now only 4 pi electrons and a positive charge is spread over carbons so aromaticity is lost.

regenerating aromaticity

heterolytic cleavage of c-h bond. so electrons in the bond go back to pi bond system.