TOPIC 17/18 - MORE ORGANIC Flashcards
What is a Chiral molecule?
A molecule with a chiral carbon atom and has 4 different groups attached.
And a chiral molecule is one that has no plane of symmetry so can get 2 non-superimposable mirror images - different properties btw enantiomers.
Chiral Carbon
A carbon atom bonded with 4 different groups attached to it (asymmetric).
A chiral center can be identified by asterisks by it. EASY TO IDENTIFY IF DRAW IN DISPLAYED FORMULA.
Optical isomers occurs by…
- come under stereoisomerism - same molecular formula but atoms are arranged spatially different.
- OCCURS when molecules have a chiral centre - get 2 non-superimposable mirror images (asymmetrical) leads to 2 possible isomers.
- draw mirror plane
When is a molecule not chiral?
- when not bonded to 4 different atoms or different groups of atoms.
- as 2 groups are same.
- there is C=C bond
- 2 chiral centres = form 4 optical isomers = 4 non- super imposable images
Chiral molecules are…
OPTICALLY ACTIVE (single enntionmers)
racemic is mixture of 2 enantiomers
What is optical activity?
The ability of a single optical isomer to rotate the plane of plane- polarised monochromatic light in molecules containing a single chiral centre.
(so angle of plane of oscillation rotates - EM rad - transverse waves - right angles)
Polarimetry - measures the amount of optical activity of a subst.
HOW does it work?
- monochromatic light source can pass through polarising filter it absorbs oscillations except in a single plane - POLARISER which CONVERTS unpolarised to plane-polarised light.
- plane-polarised light passes through sample tube if subst optically active - plane-polarised light roates.
- second polarisng light - analyser measure angle of rotation.
If plane-polarised light roatates CLOCKWISE
Dextrorotatory (C next to D)
If plane-polarised light roatates ANTICLOCKWISE
LAEVOrotatory
What is a racemic mixture?
The 50:50 mixture or equal concentrations of two enantiomers that have no optical activity.
- planar structure also due to attack on either side
- the rotation in plane of polarised light in opposite directions cause (distinguish btw stereoisomers) and Cancel out each others light rotating effect
IF NOT IN EQUAL AMOUNTS - chiral molecules only show optical activity if 1 isomer present in a greater quantity.
2 enantiomers have identical physical properties but effect rotation of plane of polarised light.
2 enantiomers have identical chemical properties but reaction with other chiral molecule differs
like 1 enantiomer can have 1 benefical effect and the other can be harmful.
SN1 reaction
- S=Substitution, N=nucleophilic , 1=unimolecular (1 species involved in Rate Determining step (1) step SLOWEST.
- Mainly in 2’ and 3’ C-X
- equal chance of being attacked by nucleophile by EITHER SIDE to:
- Forms two products
- It can form a racemic mixture as equal concentrations of opposite enantiomers are formed - NO optical act.
- tetrahedral shape -> planar
- carbocation intermediate formation after Cs+-Xs- polarizes.
What to consider in SN1
- Nucleophilic and electrophilic addition reactions always produce racemates
- STERIC HINDERANCE as more S.H , more Nucleophilic attack - increase Rate determining step.
GRAPH:
- The Ea required to form carbocation intermediate much greater than step 2 attack of nucleophile to carbocation - much faster
SN2 reaction
-S=Substituiton, N=Nucleophillic,
2=bimolecular:
- tempoarary formation Nu & C bond
- C-X bond breaking ALL IN TRANSITION STATE
- Mainly in 1’ and 2’ haloalkanes.
- Nu attack on 1 side.
- PRODUCT is an INVERSION of Reactant SO has OPPOSITE optical activity.
- Dextrot enant as REACTANTS as Laevorot enant Prod
-If the molecule is an enantiomer it will reverse the way in which it rotates the polarised light
Why 1’ haloalkane not used in SN1
- primary carbocation formed and more energetically unstable - INSTABLE requires higher level of Ea so by undergoing SN2 reaction the Ea is lowered.
Aldehyde and Ketone
- Aldehyde has C=0 at end of C chain
- Ketone has C=O in btw R chain
C=O has lower priority so give lower no. than methyl group
Can Aldehydes and Ketones form Hydrogen bonds?
No because they do not have a hydrogen attached to O,N or fluorine a highly electronegative group
only LF, chain increases, Mr and e- increases, higher density so greater polarity. - ethanoic acid > ethanol
Can Aldehydes and ketones form hydrogen bonds with water?
yes, functional group allows it as energy able to compensate for it and capable to disrupt H bond in water.
As chain length increases, solubiltiy decreases , alkyl chain incr, less energy needed
What reagent is used to identify both aldehydes and ketones?
2,4-DNP (DiNitroPhenylhydrazine) transparent
Positive result: Bright Orange Percipitate hydrazone
What reagent is used to identify both aldehydes and ketones?
Fehling’s or Benedict’s solution, Tollens’ reagent and acidified K dichromate(VI) ions = ALL OXIDISING AGENT.
- warmed BRICK RED = ALDEHYDE, BLUE = KETONE
- ammonical silver nitrate (NaOH + AgNO3 add dil NH3) - silver mirror formed for ALDEHYDE when gently warmed. REDOX react Ag+ reduced
- orange to GREEN - reduced Cr
How can we identify the carbonyl compound? From 2,4
- Filter, dry and purify the derivative 2,4-DNP
- Measure the melting point of the derivative
- Compare the value to a data booklet and check which carbonyl compound it belongs too
Identify with triodomethane CHI3
- aldehyde and ketone and some alcohols
- Has to have C=O next to a CH3 to give +ve result which is a pale yellow precip (insoluble solid) , due to reaction of I2 in alkaline sol.
reactions of carbonyl compounds with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether.
LiAlH4 OR NaBH4
REDUCING AGENT so [H]
- explosive in H20
- reduces carboxylic acids and aldehyde and ketone back into 1’ and 2’ alcohol.
so 2[H]
- Rapidly reacts impossible to stop halfway so would not be able to get aldehyde from carboxylic acid
- nitrile into primary amine
Nucleophilic addition of NaBH4
- C=O dipole susceptible to nucleophilic attack
- pi bond in C=O breaks and reactive intermmediate prod forms
- extra pair of e- on O donated to neighbouring H atom.
- source of hydride ions and forms ALOCHOL due to Reducing agent.
Nucleophilic addition of HCN/KCN
- must be acidified as :CN cannot directly react with carbonyl compounds and the C=O bond increases polarity. Only slight acid if a lot more CN- will be converted into HCN.
- HCN is highly toxic, highly volatile so used in KCN/NaCN in alkaline sol. So KCN + HCl.
- planar struct, no chiral –> source of H+ ions for Step 2. then chiral molecule forms
-Forms 2 enantiomers depending on which side CN- attacks from to form a racemic mixt - Hyrdoxy nitrile forms. - the C of nitrile counts in naming and count C number and number OH gp
- CN- ion forms
ONLY SYMMETRIC KETONE will produce a single prod with No chiral centre.
HCN = H+ + :CN- - vey HIGH pH - few H+ = step 2 slows down
- very LOW pH - more H+ = :CN- are lack in no. so step 1 slows down - interfere with reaction and prevents CN- reacting
Carboxylic acids
- COOH carboxyl gp
- despite similar Mr of COOH, OH, C=O,C=O. COOH has higher bp - 2 molecules of acid producing a DIMER (2*H bond).
and DOUBLE SIZE OF molec = incr Mr and e- - incr, van der waals
- small COOH very soluble in polar solvents
- only polar RCOO- can form H bonds
carboxylic acids made by
reduction 4[H] - 2H added and H2O
- oxid of 1’ alcohol
- it would be 2[H] if forming an aldehyde
carboxyilic acid reacts with a base
RCOOOH + NaOH
ionic salt + water
RCOONa + H2O
and Metal propanoate formed
If M-CO3 reacts then CO2 produced - THIS IS TEST FOR CARBOXYLIC ACIDS
Carboxylic acid reacts with PCl5 in COLD
not same for alcohols
RCOCl + POCl3 + HCl
acyl chloride + POCl3 + HCl
methanoyl chloride - seperated by fractional distillation
- steamy acidic fumes
Esterification
RCOOH + ROH –> C=OO + H20
ester - first part alcohol, second part carboxylic acid (ethyl ethanoate)
REVERSIBLE REACTION
- Prescence of Acid Catalyst - H2SO4 and gentle heat
- never pure and do not always go to completion
- Esters are fruity smelling
- food flavourings, perfumes, used as solvents, used as plasticisers
Acyl Chlorides
RCOCl - 2 electroneg atoms so readily attacked by nucleophile. So nucleophile addition/elimination reactions
REACTS WITH H20
at r.t
- carboxylic acid and HCl
Acyl Chlorides reacts with Alcohols
at rt
Esters and HCl
- by addition of alkyl gp of ROH
Acyl chlorides reacts with CONCEN. NH3 at rt
Amide and HCl
- addition of amide gp - NH2 forming an amino acid in a way e.g. propanamide
- HCl reacts with NH3 —> NH4Cl and the amide
Acyl chlorides reacts with Amines
RNH2 at rt
N-substituted amide and HCl
N- methylethanamide
- only 1 H given to form HCl
Hydrolysis of esters in acidic condition
heat under reflux reversible reaction to give carboxylic acids and alcohol formed.
on top of arrow write h2o
- does not go to completion
Hydrolysis of esters in alkaline conditions
(add base) SAPONIFICATION formation of soap
CH3CH2COOCH3
heat under reflux
- not heat under reflux
- goes to completion and not reversible so not reach equilibrium
- better yield
- forms carboxylate salt and and alcohol
CH3CH2COONa + CH3OH
- then carboxylate salt is added to dilute acid to form carboxylic acid.
Saponification
alkaline hydrolysis (using 3 sodium hydroxide solution) of the big esters found in animal and vegetable fats and oils.
- salt of a carboxylic acid is formed - soap
- An alcohol is also produced - glycerol
- In presence of a soap has both polar and non-polar ends, the non-polar ends of the detergent which is repelled by water interacts with the nonpolar grease. At the same time, the polar ends are attracted towards the hydrophilic molecules. The soap forms micelles in water.
Polyamides -Nylon
- made from 1,6-diaminohexane and hexanedioic acid forms a small molecule of hydrogen chloride.
- An amide link is similar but replaces the bottom oxygen by a N-H group.
- Uses in textiles for clothing and carpets, a lot of nylon is used to make tyre cords. The fibres are also used in ropes, and nylon can be cast into solid shapes for cogs and bearings in machines.
Kevlar
- The two monomers are benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene.
- they join benzene rings.
- both have an amide link
- very strong material - about five times as strong as steel, weight for weight. It is used in bulletproof vests, in composites for boat construction, in lightweight mountaineering ropes, and for lightweight skis and racquets
- due to long chains which include polar bonds (C-O and C-N) the strength of the intermolecular forces between the different chains in polyamides and proteins is enhanced by the presence of hydrogen bonding. Most importantly Kevlar molecules are parallel to each other and tightly bound.
Addition polymerisation
unsaturated molecules added together forming a polymer.
- at least C=C bond
- only 1 product formed
test for Acyl chloride?
. Add a few drops to water, test with litmus and silver nitrate solution. Litmus turns red and a white precipitate with silver nitrate.
. Add to a little ethanol and get an ester and hydrogen chloride are formed
butanoic anhydride + propan2ol -> l
methylethyl butanoate and butanic acid
amines - NH2
derived from NH3 where 1 more H atom is replaced by alkyl/ aryl group
107’ bond angle and trigonal pyramidal
NH3 - trigonal pyramidal = 1 lone pair of electrons exert a stronger repulsive force
NH4+ - tetrahedral - 2 bonding pairs repel
1’ / 2’ amine etc
bp of amine
they are much higher than alkane despite having the same e- no and shape due to their ability forming H bonds with each other
ability decr as 2’ amine as strength of dipole slightly less due to 2 R groups
3’ amine cannot form H bond as there are no H atoms attached to N
HYDROLYSIS OF NITRILES cond UNDER REFLUX, C triple bond breaks
CN
ACID COND: add H+ and H20
- carboxylic acid and NH4+ ION FORMED
ALKALI COND: add OH- and H2O
- Carboxylate ion and NH3
and ion put into dilute acid to give carboxylic acid
condensation polymerisation
formation of a polymer by the reaction of 2 different monomers in which a small molecule is also formed
- if diff functional gps OR more than 1 monomer present make a condensation polymer
e. g. polyester, nylon, terylene, kevlar
terylene
dicarboxylic + dialcohol
- clothing fibre, hard plastics
polyesters are broken down by biodegradable by hydolysis due to polarity btw polymer and are biodegradable easily broken down by moisture and gradually decompose
amide O=C:NH2 neutral unlike amines that are basic
mp is high
- form H bonds with lone pair of e- on N
- lots of potential for H binds formed
- great amounts of energy needed to break bond
- polyamide link = C=ONH formed btw diacid and diamine
due to C=O an e- withdrawing gp and pulls e- density towards O away from N - lone pair of e- on N get delocalized and more stable it is and harder to combine
[Cu(H2O)6] 2+ + 4C4H9NH2
- water is amphoteric act as acid/base donate 2 H+
[Cu(H2O)2(C4H9NH2)4] 2+ + 4H2O
ligand exchange
hydrolysis of polyesters in ACIDIC cond
forms DIOL and a DICARBOXYLIC ACID
hydrolysis of polyesters in ALKALI cond
forms a DIOL and a SALT OF DICARBOXYLIC ACID
hydrolysis of polyamides in ACIDIC cond
forms a DICARBOXYLIC ACID and a DIAMMONIUM SALT
hydrolysis of polyamides in ALKALI cond
forms a DIAMINE and a SALT OF DICARBOXYLIC ACID
amino acids
when both different functional gps attached to the same C atom = COOH and NH2 - simple AA glycine bonded with peptide bonds/amide links - they are chiral - high mp and high solubility
zwitterion
ions which have both permanent positive charge and negative charge but overall are neutral - no migration on electropherisis
= AMPHOTERIC PROPERTY
exist as this due to an internal exchange in H+ to make an internal salt
COOH is deprotonated COO-
NH2 is protonated = NH3+
in strong acid AA
the lone pair of e- on NH2 is gains a H+ and become protonated = NH3+
in alkaline cond AA
COOH gp the H+ gp is lost and is deprotonated COO-
isoelectronic point
neutral pH and its dif for diff AA, diff R gp
high I.P = more BASIC molec
low I.P = more acidic molec
synthesis of amines from haloalkanes
- ethanolic ammonia
- sealed test tube
- under pressure
- nucleophilic substit
CH3Cl + NH3 -> CH3NH2 + HCl
in CH3NH2 the N has a lone pair of e- which still acts as a nucleophile and still attack CH3Cl forming a 2’ AMINE and so on
CH3Cl + CH3NH2 -> (CH3)2NH + HCl
3’ amine = (CH3)3N
to maximise the yield of 1’ amine
you would add EXCESS NH3 and this minimises further substitutions/ side reactions if only 1’ AMINE needed.
so forms CH3NH2 + NH4Cl
amines have weaker intermolecular forces than alcohols because…
N has a lower electronegativity than O so weaker the H bonding
how can amines act as bases
the lone pair on N atom accepts a proton
how can amines act as nucleophiles
when they bond with an electron def C atom
in order strongest base what must a particular amine have
greatest e- density around N atom making it a better e- pair donor
what effect do alkyl gps have on e- density and base strength
positive inductive effect incr electron density around N = stronger base
3’ amine = insoluble = not a gd base
nitriles can be reduced into 1’ amines
using H2 at high temp and press, presence of Ni catalyst = reflux
CH3CN + 2H2 -> CH3CH2NH2 ethylamine
nitriles can be reduced to 1’ amines using strong reducing agent
LiAl4 in dry ether with acid catalyst
CH3CH2CN + 4[H] -> CH3CH2H2NH2
