UNIT 7 advanced organic chemistry

what are initiation reactions

homolytic fission under UV splits molecules creating 2 free radical molecules

what are propagation reactions

a reaction where the number of radicals is maintained. 1 molecule with a radical and 1 without will react

what are termination reactions

radicals are removed- two molecules with radicals react together

what constitutes a halogenoalkane

an alkane where at least 1 hydrogen is replaced by a halogen (group 7 element)

what are 3 examples of halogenoalkanes

tetrachloromethane (CCl4)- used for drycleaning and has a tetrahedral shape
chloroform (CHCl3)- used as an anaesthetic and has a distorted tetrahedral shape
chlorofluorocarbons (CFCs eg CFCL3)- used as refrigerants, damages ozone layer

polarity of carbon halogen bonds

they have a permanent dipole because the halogen is more electronegative than the carbon. this causes permanent dipole-dipole reactions

which will have a higher boiling point and why, trichloromethane or trichlorofluoromethane

trichloromethane is not symmetrical meaning that it has dipole dipole interactions instead of just van der waals. this means that it will have a higher boiling point than trichlorofluoromethane.

why arent alkanes reactive

C-C & C-H sigma bonds are strong
C-C bonds are non-polar
electronegativity of C & H is so similar that the C-H bond can be considered non-polar

what reaction conditions make alkanes react + examples

heat and catalysts, eg combustion and reforming

why are termination steps rare

because there are very few free radicals formed so there's much less probability that 2 will react together to remove the radicals

which will have a lower bond dissociation energy F-F or Cl-Cl and why

F-F will because it has a small atomic radius meaning that the bond length is very small so electrons repel eachother.

which is faster, fluorination or chlorination

fluorination

condtions for nucleophilic substitution

nucleophile reacts with halogenoalkane in aqueous solution

conditions for elimination

halogenoalkane forms alkene under hot ethanolic conditions. the attacking molecule always attacks the hydrogen on the carbon next to the carbon bonded to the halogen

electrophilic addition (4)

1 electrophile is attracted to the electron dense pi bond of the alkene
2 electrophile accepts the electron pair from the double bond (arrow goes from bond to electrophile)
3 a carbocation that is formed and negative ion formed
4 both of them bond together so nothing is lost (1 product)

how do you know which hydrogen the electrophile will attack

the central carbon needs to have as many alkyl groups as possible as this makes it the most stable

why do more alkyl groups mean more stability

because the carbon is an electron deficient centre so pulls the alkyl groups in making it more stable as electrons come closer

why dont fluoroalkanes react

cf bond is very strong because of electronegativity difference

what are the 2 issues with the reaction of nitrogen based nucleophiles and how are they resolved

1 the acid product formed can react with the nucleophile base
2 the amine product is also a nucleophile so can react with the original reactant

fix this by adding excess ammonia
1 this can react with the acid formed
2 more ammonia means that theres a lower concentration of the nucleophile product so the reactant is more likely to react with the ammonia

why dont cfcs go throuhg nucleophilic substitution or elimination (2)

the strenght of the cf bond and because carbon cant expand its octet

mechanism for radicals destroying the ozone layer

2O3 turns into 3O2

CF3Cl --> (UV) Cl + *CF3

Cl + O3 --> ClO + O2
ClO + O --> Cl + O2

what does ozone absorb

UV radiation

formula for benzene

C6H6

what is an aromatic compound

any compound with at least one benzene ring

what is benzene + OH

phenol

what is napthalene

two benzene rings bonded side by side

why is the formula C6H6 instead of an alkane or alkene

because bonding consists of a combination of two possible electronic configurations (resonance structures) and the bonds are halfway between double and single

how do we know that the bonds in benzene arent just alternating between double and single

because double and single bonds are different lengths and all the bonds in benzene are proven to be the same length

what type of reaction does benzene go through and with what molecule

electrophilic substitution with benzene WITH A CATALYST. DOESNT REACT WITH HBr

describe the bonding in benzene

each carbon is sigma bonded to 3 different atoms, (2 carbons and a hydrogen) leaving one carbon left in its p orbital. this causes the p orbitals to overlap forming two donut shaped areas of 6 delocalised electrons above and below the plane of the molecule.

how does the bond enthalpy of benzene prove that its more stable than kekule thought

when you add hydrogen to cyclohexene and cyclodihexene, the enthalpy increases each time. if benzene was the same structure as cyclotrihexene as kekule thought, you would expect the bond enthalpy to increase again. the bond enthalpy of benzene decreases indicating that the energy required to break the benzene bonds is a lot higher than the energy to form the bonds. this shows that it's stable with string bonds.

what are aromatic compounds known as

Arenes

how do you name an arene (2)

1 if there's only 1 functional group, you dont need to number it
2 otherwise name it from the first functional group (alphabetical)

what is it called when a benzene ring loses a H and bonds to another group

a phenyl (nothing to do with phenol)

what was the main source of benzene in the 1940s

coal

what are the two ways you can get benzene

1 catalytic reforming where C6H14 is heated to 900 degrees with a catalyst to remove the extra hydrogens and create C6H6 + 4H2
2 manufacture from naptha where C6H14 is heated to 400 degrees with a catalyst to create C6H12 + H2 and then again to from C6H6 + 3H2

give 6 examples of electrophiles

Acids, carbocations, Br2 Br+, Cl2 Cl+

what is needed in electrophilic substitution and why

a catalyst because the benzene bonds are strong enough that the permanent dipole isn't enough to affect it

what needs to happen before electrophilic substitution (+ eg)

the carbocation needs to be formed. eg in chlorination, the AlCl3 accepts a pair of electrons from 1 chlorine and the other chlorine donates a pair of electrons to break the Cl2 bond. this forms AlCl4- and Cl+

how is the catalyst regenerated

the H+ from the benzene bonds to 1 of the Cl molecules from the AlCl4- creating HCl and AlCl3 again

Why is nitrobenzene important

it can be reduced to form an amine which is used in a lot of drugs

what two things can you get from benzenesulfonic acid and how

1 when you add cold dilute NaOH you get benzene sodium sulphate
2 when you add hot concentrated NaOH you get sodium phenoxide (you can add acid to this to get phenol)

describe addition of hydrogen to benzene

Hydrogen is added to the benzene with a nickel catalyst. this reaction is impossible to stop after 1 hydrogen is added so benzene will keep gaining hydrogens

describe addition of chlorine to benzene

chlorine bubbled through refluxing benzene in UV light (this causes addition instead of electrophilic substitution)

what happens when you add an alkyl group to benzene

it activates it because because they are less electronegative than the benzene so can increase the electron density in the ring. this makes it more reactive

what happens when you add a halogen to benzene

it deactivates it because the halogen is more electronegative so does not increase the electron density in the ring making it less reactive

what happens when you add NO2 to benzene

it also deactivates it because the oxygen is more electronegative so no more electron density will be added to the ring. it also has no lone pairs on the oxygen to donate to the ring. this means its less reactive

Why are phenols more reactive than benzene

the lone pair on the oxygen can be donated to the benzene ring, increasing the electron density and making it more reactive (activating it)

what is a hydroxyl group

OH-

what is an amine

NH2

what reaction does phenol go through and in what conditions

electrophilic substitution with a halogen in the presence of a catalyst (eg AlCl3)

what is formed in the electrophilic substitution chlorination reaction with phenol

2,4,6-trichlorophenol or TCP

why does nitration of phenol not require sulfuric acid when benzene does

because phenol is much more reactive than benzene due to the lone pair on the OH increasing the electron density of the benzene ring, activating it

what is a phenylamine

a benzene ring bonded to NH2

is phenylamine reactive and why

yes because the lone pairs on the nitrogen add to the electron density in the pi system, (activating)

are alkyl groups or lone pair groups more activating and why

alkyl groups are weakly activating because they are polar so do not donate electrons to the pi system, the electrons are just closer in to the pi system because alkyls are less electronegative. the lone pair groups donate a lone pair to the ring so are strongly activating

does the chlorination of methylbenzene require a catalyst and why

yes because even though the methyl group is activating, it's still only weakly activating

what 3 things does a phenol act as

weak acid, proton donor and nucleophile

how do you test for phenols

when it reacts with iron (III) chloride it forms a violet coloured solution

what is a carbonyl group

carbon double bonded to an oxygen

what is an aldehyde

carbonyl group bonded to a hydrogen and an r group

what is a ketone

carbonyl group bonded to 2 r groups

how are aldehydes ketones and carboxylic acids formed

aldehydes- oxidising a primary alcohol
ketones- oxidising a secondary alcohol
carboxylic acids- further oxidising an aldehyde

what needs to be present when forming aldehydes and ketones

acidified potassium dichromate (H+/K2Cr2O7)

how are aldehydes and ketones named

aldehyde- ends in al
ketone- ends in one

what reaction do carbonyl compounds go through and why

the c=o double bond is polar due to the electronegativity of the oxygen meaning that the carbon is an electron deficient centre (delta positive) so nucleophiles attack it

why does the formation of an aldehyde have to happen in a distillation apparatus

so that the newly formed aldehyde is able to escape, otherwise it would be further oxidised to form a carboxylic acid

what happens to tertiary alcohols when they are oxidised with H+/K2Cr2O7

no reaction because there are no hydrogens on the carbon bearing the C=O double bond

what is bradys reagent used for

it reacts with carbonyl groups and forms an orange precipitate so is used to test for carbonyl compounds

how is bradys reagent used

the compound can be recrystalised and the melting points can be compared to different known carbonyl compounds

how can you distinguish between aldehydes and ketones

in reduction, aldehydes will become primary alcohols and ketones will become secondary alcohols. in oxidation aldehydes will become carboxylic acids and ketones will not react

what happens if you leave you dont keep aldehydes in an airtight container

they can oxidise to carboxylic acids because they're extremely susceptible to oxidation

what is fehlings reagent and how is it used to test things

Cu2+ used to test for the presence of aldehydes. Cu 2+ oxidises the aldehydes and becomes Cu2O and goes from blue to brick red. its alkaline and ketones dont react because they cant be oxidised

what is tollens reagent

A solution of silver nitrate in aqueous ammonia. the Ag+ ions get reduced to silver metal in the presence of an aldehyde but not ketones

how can you determine whether a substance is primary secondary or tertiary

count the carbons attached to the carbon baring the significant group (OH for alcohols and halogens in halogenoalkanes)