Organic Synthesis OCR Chemistry A

Frage	Antworten
Homolytic fission	each atom in a covalent bond gets one electron, producing free radicals
Heterolytic fission	one atom gets both electrons when a covalent bond is broken.
free radicals	reactive species which possesses an unpaired electron
What type of reaction occurs between a halogen and an alkene?	Free radical substitution
Type of reaction, Reagents, conditions and stages of free radical substitution to form a haloalkane	Type of reaction: Free Radical substitution Reaction taking place: Halogenation Reagents: Alkane + halogen (eg. Br2) Conditions: UV light Three stages: initiation reactions, propagation reactions, termination reactions
what is a double bond made up of?	A sigma-bond and pi-bond.
What makes alkenes more reactive?	The pi-bond in the double bond is formed from the overlap of p-orbitals. These pi-bonds are much weaker than sigma-bonds due to the spread of electron density above and below the molecule, so there's weaker electrostatic attraction between the nuclei and electrons. This means that it is more likely to be attacked by electrophiles.
Electrophile	electron-pair acceptor
Type of reaction alkenes go through	Electrophilic addition reactions Image: Electrophilic Addition Reaction (binary/octet-stream)
Markownikoff's rule	H adds to the carbon with the most H atoms already attached as this is the more stable carbocation.
What makes a carbocation more stable?	more alkyl groups = more stable because the alkyl groups are electron releasing and reduce the charge on the positive carbon atom, stabilising it.
Test for C=C double bond	Shake with orange bromine water. positive = goes colourless (because Br is added across the double bond to form a dibromoalkane)
production of alkanes from alkenes	Reagents: Alkene + Hydrogen Conditions: Ni catalyst, 150 degrees Celcius Reaction taking place: Hydrogenation
Formation of alcohol from an alkene	Reagents: alkene, steam Conditions: 300 degrees Celsius, 60-70 atm, H3PO4 catalyst Reaction taking place: Hydration Type of reaction: electrophilic substitution Image: Image (binary/octet-stream)
products of complete combustion of alcohols	carbon dioxide and water
Formation of haloalkanes from alcohols (reagents, conditions, type of reaction)	Type of reaction: substitution reaction Reaction taking place: Halogenation Reagents: alcohol, halide ion Conditions: acid catalyst (H2SO4)
Dehydration of alcohols	Type of reaction: Elimination reaction Reagents: alcohol, conc. H2SO4 Conditions: Heat under reflux Products: Alkene and water Image: Image (binary/octet-stream)
Oxidation of Primary Alcohols	Reagents: alcohol, potassium dichromate solution [O] Conditions: Distil for aldehyde, reflux for carboxylic acid Observation: orange dichromate(VI) ion is reduced to the green chromium(III) ion, Cr3+ Image: Image (binary/octet-stream)
Oxidation of Secondary Alcohols	Reagents: alcohol, potassium dichromate solution [O] Conditions: Distil for ketone Image: Image (binary/octet-stream)
Oxidation of Tertiary Alcohols	Cannot be oxidised Image: Image (binary/octet-stream)
Haloalkane	An alkane with halogen atoms
Nucleophile	electron-pair donator
Reactivity of haloalkanes	Halogens are much more electronegative than C, so the C-X bond is polar. The delta+ C is electron deficient, so attracts nucleophiles to undergo nucleophilic substitution reactions.
Hydrolysis of haloalkanes	Type of reaction: Nucleophilic Substitution Reaction taking place: Hydrolysis Reagents: haloalkane, NaOH Conditions: warm aqueous alkali, reflux Product: Alcohol Image: Image (binary/octet-stream)
Rate of substitution reactions with haloalkanes	As polarity of the halide decreases (down the group), C is less attracted to nucleophiles, so the rate decreases. However, as bond strength decreases (down the group), the bond breaks more easily, so rate increases. overall order (fastest to slowest): iodoalkane, bromoalkane, chloroalkane, fluoroalkane.
Formula of benzene	C6H6
Comparison of Kekule's model and the delocalised model for benzene	Kekule: Alternating single and double bonds. P-orbitals overlap between two C atoms Localised electron density in pi-bonds. Delocalised: P-orbitals of all 6 C atoms overlap to create a pi-system Pi-system made up of two rings of delocalised electrons above and below the plane of the molecule. Equal length C-C bonds
Evidence for the delocalised model of benzene	1. Normally, the length of C-C bond is 154pm and C=C bond is 134pm, but in benzene, all C-C bonds are 140pm. 2. Enthalpy change of hydrogenation for cyclohexene is -120 kJ/mol so is Kekule was right, it would be 3 times this for benzene (-360 kJ/mol) but it is actually -208 kJ/mol. This is less exothermic than expected. 3. Benzene should react with bromine water at room temperature like alkenes, but it is actually only possible in the presence of a halogen carrier or high temp. So it is more stable than an alkene.
Aromatic compound	compounds containing a benzene ring
What makes benzene more stable than alkenes?	In benzene, electrons are delocalised, in alkenes they are localised between two carbon atoms. This means that benzene has a lower electron density than alkenes, so benzene is unwilling to undergo addition reactions because it is too stable.
What reactions to arenes undergo?	Electrophilic substitution reactions. (A hydrogen atom is replaced by an electrophile) Image: Image (binary/octet-stream)
Halogen Carrier	They make compounds into stronger electrophiles. They accept a lone pair of electrons from a halogen atom on an electrophile and as the lone pair is pulled away, polarisation in the molecule increases, forming a carbocation. This makes the electrophile stronger, allowing halogens to substitute into the benzene ring. Image: Image (binary/octet-stream)
Halogenation of Benzene	Type of reaction: Electrophilic Substitution Reaction taking place: Halogenation Reagents: Halogen, halogen carrier Conditions: halogen carrier catalyst (AlCl3) Product: halobenzene
Friedel-Crafts alkylation	Form C-C bonds by putting an alkyl group onto a benzene ring using a haloalkane and halogen carrier, and reflux. Image: Image (binary/octet-stream)
Friedel-Crafts acylation	Substitutes an acyl (C=O) group for a H on benzene. Reflux benzene with an acyl chloride and halogen carrier to produce phenylketones. Image: Image (binary/octet-stream)
Production of nitrobenzene	Reagents: Conc. nitric acid, benzene Conditions: warm, conc. H2SO4 catalyst, for mononitration temp needs to be below 55 degrees, otherwise multiple substitutions. Image: Image (binary/octet-stream)
Phenol	C6H5OH Benzene ring with alcohol group attached Image: Image (binary/octet-stream)
Why is phenol more reactive than benzene?	Lone pair of electrons from oxygen p-orbital overlap with the delocalised ring of electrons in the benzene ring, so it is partially delocalised into the pi-system. This increases electron density of the ring, making it more likely to be attacked by electrophiles.
Where do electron donating groups direct substitution? Give examples of electron donating groups.	Electron donating groups include -OH, and -NH2. They direct substitution to the 2-, 4-, and 6- carbons.
Where do electron withdrawing groups direct substitution? Give examples of electron withdrawing groups.	Electron withdrawing groups include -NO2. They direct substitution to the 3- and 5- carbons because it withdraws electron density at the 2-, 4- and 6- carbons, so they can't react with electrophiles.
Reaction of phenol with bromine water	Reagents: bromine water (Br3) Type of reaction: electrophilic substitution products: 2,4,6-tribromophenol and HBr observation: orange to colourless Image: Image (binary/octet-stream)
Nitration of phenol	Reagents: dilute nitric acid Type of reaction: electrophilic substitution Products: 2-nitrophenol, 4-nitrophenol Image: Image (binary/octet-stream)
Phenol with bases (NaOH)	Phenol is weakly acidic so will undergo typical acid-base reactions. Reagents: NaOH Conditions: room temp Products: sodium phenoxide and water Image: Image (binary/octet-stream)
Carbonyl compounds	Aldehydes and ketones Contain C=O group
Reduction of aldehydes and ketones	Type of reaction: nucleophilic addition Reagent: reducing agent [H] NaBH4 (sodium tetrahydriborate (III) or sodium borohydride), then water Products: alcohol

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Organic Synthesis OCR Chemistry A

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