key concepts for MCQ test Flashcards
1
Q
can sigma bonds rotate freely?
A
yes
2
Q
can pi bonds rotate freely?
A
no
3
Q
what type of spins do electrons have in covalent bonding?
A
opposite
4
Q
bond dissociation energy
A
measure of how strong a bond is.
when you bring 2 atoms together there is a release of energy, so to break the bond you need to put energy back in
5
Q
electronegativity
A
ability of an atom to attract a shared paired of electrons/ electron density towards itself. It is a chemical property
6
Q
what is electronegativity determined by?
A
- nuclear charge (more protons = more pull of electrons to nucleus)
- location of electrons in orbitals
- electron shielding
7
Q
inductive effect
A
- effects of electronegativity on charge distribution of a molecule
- dipoles arise due to differences of electronegativity in atoms
8
Q
groups that are more electronegative than a carbon atom (delta negative)?
A
- halogens
- NO2
- OH
- SH
- SR
- NH2
- NHR
- NR2
- CN
- carboxylic acids
- aldehydes
- ketones
9
Q
groups that are LESS electronegative than carbons (delta positive)?
A
- alkyl R groups
- metals (Mg)
10
Q
why does the pka decrease when more chlorines are added to a carbon next to a carboxylic acid?
A
- cl is electronegative
- so electron density is being pulled towards the chlorine
- this causes the negative charge to stabilise across the anion
- decreasing the pKa
- so molecule with 3Cl is more stable due to e- being pulled towards all 3
11
Q
what are resonance forms?
A
electrons are delocalised into the model (resonance = e- arranged differently)
12
Q
mesomeric effect
A
lone pair next to a pi bond
(overlap of lp of e- in a p orbital with adjacent pi bonding systems)
13
Q
electron withdrawing substituents and the mesomeric effect
A
-M
- electrons are withdrawn from the pi bond.
- orbital overlap causes the p orbital be low in electron density,
- decrease in pKa, stronger acid, more stable.
14
Q
electron donating substituents and the mesomeric effect
A
+M
- lp donate electrons to the pi system.
causes pKa to increase so weaker acid.
so it is less stable
15
Q
what does it mean if the pH of a sol is equal to the pKa?
A
solution is 50% protonted
16
Q
what does the alkyl group in an alkyl amine do?
A
- alkyl group is electron donating (+M)
- so pushes electrons towards the N atom
- N becomes more negative
- so more attractive to H+
17
Q
why can the lone pair on a N atom of phenylamine not pick up a proton?
A
- lone pair is sp2 hybridised
- overlaps with aromatic ring
- reduced availability to accept proton as the lone pair is delocalised into ring system
- phenyl amine = weaker base
18
Q
why can the lone pair on N atom of pyridine pick up a proton?
A
- lone pair is pointing away from the pi system
- lp is in an sp2 orbital
- can accept proton
- as lp is available and not delocalised into ring system
- pka still low so weak base
19
Q
pKa of amides
A
- amides have a very low pka
- lone pair on N is sp2 hybridised
- lp in p orbital overlaps with the carbonyl =O bond (pi bond)
- so lp not available to accept proton
20
Q
in order to protonate an amide what must you have?
A
VERYY low pH
21
Q
how do carbonyls react?
A
- polar oxygen is electronegative
- pulls e- density towards it
22
Q
what determines how electronegative the =O of a carbonyl is?
A
the functional groups attached to it
23
Q
what do strong electron donating groups (e.g OH) do to carbonyls?
A
makes the delta + on carbon smaller (reduces difference in electronegativity of c and o), so makes reactivity towards Nu weaker
24
Q
what do strong electron withdrawing groups (acyl chlorides) do to carbonyls?
A
makes delta + on carbon bigger (bigger difference of electronegativity of atoms) so reactivity towards Nu increases
25
name all functional groups you didn't know before
- ether
- thiol (SH)
- lactone (ester in a ring)
- disulphide bridge (R-S-S-R)
- lactam (amide in a ring)
- beta lactam
26
what is the alpha position?
position next to carbonyl
27
describe amides
- mesmeric effect due to lone pair on N next to the pi bond (carbonyl)
- lp not available
- NOT basic
28
hybridisation of an ether?
sp3
29
hybridisation of a ketone?
sp2
30
hybridisation of an ethyl ester?
sp2
31
hybridisation of an a primary amine?
sp3
32
hybridisation of a primary amide?
sp2
33
hybridisation of a thiol?
sp3
34
pka of HCl?
-7
35
pka of CH4?
about 50
36
pka of carboxylic acid?
about 5
37
pka of phenol?
~ 10
38
pka of alcohol?
~ 15
39
keto vs enol
keto: the one with the ketone
enol: the one with the alkene and alcohol
bc the H jumped over
40
carbocycle
rings made of C atoms
41
saturated
no double bonds
42
heterocycles
contains heteroatoms (O,S,P,N)
e.g cyclic ethers
43
epoxide/ oxIrane (heterocycle) - example of cyclic ether
2 membered carbon ring with one oxygen in it
44
oxEtane - cyclic ether
3 membered carbon ring with one oxygen in it
45
cyclic amines (HINT: IDINE)
- azIridine: 2 membered carbon ring with NH in it
- azEtidine: 3
- pyrrolidine: 4
- piperidine: 5
46
unsaturated version of pyrrolidine (4 C) - 2 double bonds
pyrrole
47
unsaturated version of piperidine (5 C) - 3 double bonds
pyridine
48
2 factors in ring formation + activity
1. entropy: bigger ring size = ends less likely to meet = ring less likely to form = smaller rigs favoured
2. ring strain
49
what is ring strain?
- smaller rings have smaller bond angles that deviate from the primal bond angle that comes from hybridisation
- smaller rings disfavoured
50
rate of formation of ring sizes?
5 > 6 > 3 >> 4
- 5 membered easier to form than 6
- 6 is faster than 3
- 3 is a lot faster than 4
51
reactivity of 5 + 6 membered rings?
- stable
- due to little ring strain
52
reactivity of 3 + 4 membered rings?
- susceptible to ring opening
53
conformational isomer definition
compounds with different arrangement of atoms from BOND ROTATION (so occurs in sigma bonds)
54
staggered conformation isomer
- lower in energy
- C-H bonds are further away
55
eclipsed conformation isomer
- higher in energy
- C-H bonds closer
56
axial vs equatorial
axial = higher in energy
equatorial = lower in energy
57
configurational isomer definiton
compounds with different arrangement of atoms from BOND BREAKING (occurs in double bonds)
58
geometric isomers
E/Z, cis trans
59
optical isomers
non superimposable mirror images of each other (enantiomers), identical physical properties, rotate PPL in opposite but equal directions
60
chiral molecule
no plane of symmetry within
61
achiral molecule
plane of symmetry within
62
PPL
- light source
- light goes in all directions
- so you put filter
- light goes in one orientation
- see observed rotation
63
anticlockwise vs clockwise PPL
anticlockwise = l or -
clockwise = d or +
64
how to find alpha D?
observed rotation / pathlegth (dm) x conc (g/mol)
REMEMBER THE PLUS OR NEG SIGN
USE THIS TO FIND MIXTURE OF SAMPLE WITH NUMBER LINE
65
cm to dm
divide by 10
66
racemic mixture and its alpha D
50:50 mixture of 2 enantiomers
- alpha D is 0 as PPL is cancelled out
67
achiral compound alpha D
0 as PPL only occurs in chiral compounds
68
enantiomeric excess (also the number line stuff)
excess of one enantiomer over another.
ee= x-y/ x+y (use the percentages of + or - enantiomer for this)
69
what 2 things go on the number line?
- the % of + or - enantiomer (depends on what answer you got for alpha d)
- alpha d of the pure sample
70
do lone pairs count in assigning s and r?
NO
71
clockwise vs anticlockwise for S and R assigning
s = anti
r = clockwise
NOTE: if lowest group isn't wedged THEN THE ORIGINAL ANSWER NEEDS TO BE SWITCHED
72
what does keto-enol tautomerisation do?
destroys stereogenic centre
73
enantiomers that can racemise?
thalidomide and ibuprofen via tautomerisation
74
enantiomers that can't racemise?
citalopram and escitalopram
75
enantiomers vs diastereoisomers
enantiomers = RS and SR, RR and SS
distereoisomers = RS and RR, RS and SS
76
epimers
when disteroisomers differ at only one sterogenic centre (RRRS)
77
good leaving group meaning?
can stabilise a negative charge (e.g Cl)
78
SN2 reaction (nucleophilic substitution)
- substitution nucleophilic bimolecular
- Nu- goes via 180 degree attack
- via transition state
- inversion of stereochemistry
- 2 things come together
79
factors for SN2 reaction?
- good, small Nu-
- primary and secondary substrates (1/2 alkyl groups only) otherwise hard for Nu to attack
80
SN2 reaction energy profile
- reactants higher than products
- one big increase (Ea) for the transition state
- exothermic reaction (energy lost to surrondings when bonds are broken)
81
SN1 reaction (nucleophilic substitution)
- the leaving group just leaves in the first step no need for Nu -
- Nu - attacks in the carbocation intermediate stage due to empty p orbital (which is trigonal planar sp2)
- forms product
82
factors favouring SN1
- good leaving group
- weak and large Nu
- stabilised carbocation
83
SN1 energy profile
- reactants higher than products
- LG leaving is highest peak as most difficult step (RDS)
- Nu attacking ring is slightly lower energy than this
- exothermic
84
E2 reaction (nucleophilic substitution)
- elimination bimolecular
- base removes proton of an alkane
- so the electron in bond moves across
- forms an alkene
- so LG has to leave
(base attacks proton in plane of LG)
85
E1 reaction (nuc sub)
- elimination unimolecular
- LG just leaves
- so carbon becomes positively charged (carbocation intermediate)
- base with negative charge removes proton
- electrons in bond move across
- form alkene
86
most reactive to least reactive carbonyls
1. acyl chloride
2. acid anhydride
3. aldehyde
4. ketone
5. ester
6. amide
87
how to make acid anhydride?
CA + CA
88
where do inductive effects occur?
through sigma bonds (so oxygen in carbonyl pulls e- out of c via sigma bond)
89
what are protons at the alpha position?
acidic
90
why are acyl chlorides more reactive than acid anhydrides?
- due to LG ability
- Cl- is better leaving group + more reactive, produces more stable acid HCl
- pka is -7 (stronger acid)
- carboxylate ion not as good LG, pka = around 5 of ethanoic acid
so smaller pka = better LG
91
why are aldehydes more reactive than ketones?
- inductive effects
- ketone has 2 inductive effects, which make the electronegativity difference smaller between c and o, so less electrophilic + reactive
- aldehyde has 1 inductive effect, so carbon is more delta positive
92
why are esters more reactive than amides?
oxygen is more electronegative than N so it pulls more electron density from the carbonyl carbon, so esters more reactive
93
why are esters and amides not very reactive?
- due to mesomeric effects
- lp of electrons next to pi bond
- pushes electrons back to carbonyl carbon, so more electronegative, less electrophilic
94
rotamers
- mesomeric effect causes double bond characteristics in C-N bond
- restricted rotation
- causes rotamers
95
carbonyl + H2O (without a leaving group)
1. tetrahedral intermediate forms
2. proton transfer
3. you form a hydrate (2 alcohol functional groups attached)
96
is eqm heavily acetone or its hydrate side?
acetone (major product)
97
is eqm heavily on formaldehyde or its hydrates side?
hydrate (major)
as the aldehyde is a strong electrophile so accepts e-
98
is eqm heavily cyclic ketone or its hydrate side?
hydrate (major) as it is sp3 so 109 degrees. due to steric it wants to be 120 degrees, 109 closer than 60
99
carbonyls + alcohol R-OH (without LG)
1. use acid catalyst to protonate the =O bond (makes it + charged)
2. forms tetrahedral
3. remove the H+ to form hemi ketal/ acetal
4. H+ added again but onto the OH to form OH2
5. C=O reforms and H20 is leaving group
6. O becomes + charged again as R group attached to it
7. R-OH attacks again
8. forms tetrahedral again but with new R-OH on it
9. called ketal or acetal
100
carbonyl + primary amine (without LG)
forms iminium which forms immune after being deprotonated
101
biosynthesis of a.a
pyridoxamine + pyruvic acid undergo condensation to form imine, double bond moves, water added, forms alanine
102
carbonyl + secondary amine (with LG)
forms iminum which forms enamine
103
how to make an ester with acyl chloride?
- acyl chloride + alcohol
- tetrahedral intermediate
- o- forms double bond
- so cl- leaves
- ester made
- very reactive no need for cat ha
104
how to make an ester with carboxylic acid?
- CA + alcohol
- acid catalyst to protonate to make it more electrophilic
- proton transfer
- O reforms
- H2O leaves
- proton on =O leaves
- ester formed
105
how to make an amide with acyl chloride?
acyl chloride + primary amine
106
CA + amine makes?
acid + base makes salt
carboxylate ion and NH3 species
107
how does nature make amide bonds from CA?
- thioesters
- acetyl CoA
- acyl phosphate
108
how is glutamate made?
- the CA is phosphorylated (ATP to ADP)
- this reacts with ammonia
109
acid hydrolysis of ester
- =O protonated to make more electrophillic
- proton transfer with this product
- lose H+ to remake =O
- make CA and OH
110
basic hydrolysis with ester
- base attacks carbonyl carbon
- OR becomes LG and leaves when =O reforms
- forms CA + OR-
- but irreversible reaction happens where it becomes carboxylate ion
111
acid hydrolysis of amine
- electrophile made more electrophullic by protonation
- makes O positively charged
- water attacks
- proton transfer
- NH3 becomes LG when O reforms
- H from protonation is lost
you make CA and NH3 / NH4+
112
basic hydrolysis of amine
- OH attacks (slow)
- NH2 leaves when O reforms
- very slow due to bad LG
- also forms irrevisble carboxylate ion
113
resonance vs tautomer
resonance = electrons arranged differently
tautomer = atoms arranged differently
114
keto / enol
constitutional isomers, diff compounds, diff structure, same molecular formula
115
acidity of alpha proton is related to?
order of reactivity of carbonyl
116
why is acyl chloride > ketone > ester > amide ?
acyl chloride has bigger inductive effect due to bigger delta plus so alpha proton is more acidic (pka ~ 15)
117
pka of ketone
~ 20
118
pka of ester
~25
119
pka of amide
~ 30
120
why is do we draw oxyanion and not carbanion enolate?
oxygen more EN than carbon so O- more stabilised, so C more reactive.
121
describe double bond
region of high electron density, nucleophilic, isolated (not conjugated)
122
when alkene under electro add is 2-chloropropane or 1-chloropropane not observed?
1-chloro
-regioselectivity
- regioisomerism
123
why is 1-chloropropane not observed?
- primary carbocation intermediate
- not very stable
124
why is secondary carbocation preferred?
1. inductive effects: alkyl groups are weakly electron donating so push e- to carbon, causing smaller delta negative. secondary has 2, primary has 1
2. hyperconjugation: orbitals overlap, which has stabilising effect
125
allylic carbocation
- double bond moves across to stabilise + change
- so + charge is delocalised over both carbons
- increases stability
126
issues with original benzene model
1. c-c bond lengths in benzene are not usually size of c-c single bonds
2. all bond lengths are same
(resonance explains this)
127
describe the correct benzene model
delocalised pi bond system above and below plane, sp2 120 degrees
128
define aromaticity
electrons are delocalised over whole CONJUGATED systems
129
define conjugation
alternating double, single, double bond
130
rules to assign something as aromatic
1. cyclic
2. flat (trigonal planar sp2)
3. conjugated
4. 6 pi electrons (remember if lp is part of ring system it counts)
131
what do aromatics undergo?
electrophilic substitution
132
aromatic + halogen (Cl2)
- requires lewis acid
- AlCl3
- e.g chlorobenzene
- halogenation
133
aromatic + acyl chloride
- Lewis acid
- alcl3
- forms the ketone on the ring
- acylation
134
aromatic + haloalkane
- Lewis acid
- alcl3
- alkylation
135
aromatic + H2SO4
- forms SO3H
- sulfonylation
136
aromatic + conc HNO3 + conc H2SO4
makes nitrobenzene
137
ortho, meta, para directing groups
ortho = 1,2
meta = 1,3
para = 1,4
138
if an R group on a benzene ring is strongly electron donating what does this mean? e.g NH2, NHR, NR2, OH, OR
- electrons are pushed into the Nu making it more nucleophilic
- 1,2 and 1,4 activating
- ortho and para directed
139
if an R group on benzene ring is weakly electron donating what happens? e.g methyl, ethyl, phenyl
- 1,2 and 1,4
- ortho and para
140
if an R group is strongly electron withdrawing what happens? e.g ketone, ester amide, nitrobenzene, nitrile
- electron density pulled out of Nu, so less nucleophilic
- directs meta 1,3
141
if an R group is weakly electron withdrawing what happens? e.g halogens
ortho 1,2 and para 1,4
142
SnAr
- nucleophullic aromatic substitution to identify a.a structure
- --R in ring but you don't know where substituents are
-
143
what is used to identify a.a sequences
sangers reagent (specifically a.a at n terminus)
144
what is an alpha, beta unsaturated ketone?
- enone
- electrophillic so toxic
- alpha position next to ketone pi bond has double bond
- could also have a,b unsaturated ester, amide, aldehyde
- conjugated
- has 2 electrophilic positions so Nu can attack at either
- can form allylic cation
145
direct addition (1,2 addition)
- nu attacks where the c=o bond is
- if LG present you get the product with c=o in it
- if LG not present you get the alcohol
146
conjugate addition (1,4 addition . Micheal addition)
- nu attacks at 4 position where c=c is
- acid used to turn alkene into alkane
147
enone reactivity
- acyl chloride reacts readily 1,2
- amide reacts readily 1,4 as the actual amide isn't that reactive, so the c=c is chosen over it
148
glutathione
delivery system of thiol used to detoxify things in body, e.g paracetamol
