Substitution reactions Flashcards
Why do alcohols and ethers not undergo substitution reactions?
OH groups are not displaced
- reaction fails
- OH is a strong base and a bad leaving group
What are good and bad leaving groups?
bad leaving groups
- strongly basic or derived from weak acids
good leaving groups
- derivatives of strong conjugated acids
What are properties of SN2 reactions?
rate = k [alkyl halide] [nucleophile] - second order reaction does not have an intermediate - has a transition state = 5 valance state, highest energy state concerted mechanism = one step process - no carbocation intermediate favours primary structures on a chiral centre it makes an inversion of stereochemistry - nucleophile attacks from the back
What are the properties of SN1 reactions?
rate = k [alkyl halide]
-first order reaction
has a carbocation intermediate
- trigonal planar = 120, empty p orbital
stepwise mechanism = two step process
favours tertiary structures
forms a racemic mixture when attacking a chiral carbon = equal R + S
- nucleophile attacks from behind and in front = attacks on the opposite side to leaving X group
What is a nucleophile?
donate pair of electrons
- Lewis base
What type of nucleophile do SN2 and SN1 reactions require? What type of solvent do SN2 and SN1 reactions require? What are the other requirements? Why do they need these?
SN2
- require a good nucleophile = charged
- require a high concentration of nucleophiles
- require polar aprotic groups solvent = do not have OH, cannot donate a H+ group
SN1
- require a poor nucleophile = uncharged
- strong nucleophiles are too reactive to allow a carbocation to form
- require polar protic group solvent = have OH, can donate H+
What are good and poor nucleophiles?
good nucleophiles
- charged molecules = hydroxyl ion, cyanide ion
poor nucleophiles
- non-charged nucleophiles = ethanol, water
What type of compound do SN1 and SN2 reactions favour?
SN1
- favour tertiary structures
SN2
- favour primary structures
Why are aldehydes, ketones and carbonyl groups more reactive than alkenes? How can it be proven?
carbonyls are the most reactive functional group
- pi bond can be broken easily
reactivity can be proven by showing the resonance structures or the dipole moment
What are the properties of aldehydes?
associated with flavourings and smells/odour
methanal is water soluble
- exists in solution as formalin
- preservative in foods = meat
benzaldehyde = smell/flavour of almonds trans-cinnamaldehyde = flavour of cinnamon thiopropionaldehyde = lachrymator from chopped onions
What is the DNP test for aldehydes and ketones? What is the hydrazine test? What do they form? What type of reaction was it?
reacts with DNP = 2,4-nitrophenylhydrazine
- forms 2,4-nitrophenylhydrazone
reacts with hydrazine
- forms hydrazone of acetone
form crystalline hydrazones
condensation reaction = elimination of water
What is the result of hydrating a carbonyl compound? What are the requirements?
hydration of a carbonyl compound forms an alcohol
- require a base or acid catalyst
What are gem diols? Why are they unstable? What is the effect of unstable gem diols?
type of alcohol where two OH groups are attached to the same C atom
gem diols are unstable
- two oxygen atoms on the same carbon leads to more lone pair-lone pair repulsion
- C=O bond is more favorable than two C-O bonds
- readily reverse between carbonyl and alcohol
What are the reasons for stable hydrates of methanal, chloral, and ninhydrin forming?
electronegative atoms with the compounds can form hydrogen bonds with each other
- carbocation is more stable or hydrate is stabilized by hydrogen bonding
- makes it more stable and less reactive
What is chloral hydrate used for? What is ninhydrin used for?
it is a sedative
- added to alcoholic drinks
was first a hypnotic anti-depressant
- put patients to sleep
used in forensic science for finger printing
- reacts with amino acids
