Hi UltimaOnline,
Your cogent explanations are amazing - thank you very much.
:D
1.
Which of the following will not be produced when 1-chloropropane is
heated in ethanolic sodium hydroxide?
CH3CH2CH2ONa
CH3CH2CH2OCH2CH3
Answer: A.
Remarks: How is B produced during the reaction?
2.
http://img.photobucket.com/albums/v700/gohby/Chemistry/compoundy_zpsd5n21kuf.jpg
Remarks: Choice 3 is correct. Is it due to electrophilic
substitution with phenylamine, even though the amine is not a
primary amine. Students would know that bromine water undergoes
electrophilic substitution with phenylamine - would they be
required to infer that it would undergo the same reaction even
where the amine is not a primary amine?
3.
http://img.photobucket.com/albums/v700/gohby/Chemistry/Cyanohydrin_zpsp7p1b7v6.jpg
Remarks: How do I know if C is correct? How do I know that the
cyanohydrin produced does not undergo an SN1 reaction and reacts
with ammonia thereafter to form an amine?
4. Re hydrolysis of halogenoalkanes/acyl chlorides/esters/amides
(sorry this is long!):
(i) What affects the rate of hydrolysis - the strength of the C-X
bond or the extent of δ+ on the C atom?
In alkyl chlorides, the rate of hydrolysis is dependent on the
strength of the C-X bond and not the polarity of the C-X bond.
However, when we are juxtaposing the rate of hydrolysis between
different groups, such as acyl chlorides and (halogenoalkanes, for
instance), it is mentioned that acyl chlorides are readily
hydrolyzed due to the electron withdrawing O and Cl atoms
intensifying the partial positive charge on the C atom.
(ii) How do I compare the rate of hydrolysis across different
groups (esters, amides, acyl chlorides, phenoxide, alkoxides,
halogenoalkanes)? I know esters and amides hydrolyse slower as
compared to acyl chlorides from the conditions required to
hydrolyse them, but what is the basis of the different rates of
hydrolysis?
Glad to be of help ;)
Q1. SN2 nucleophilic attack by the ethanol solvent molecule (ie. a
competing nucleophile).
Q2. Yes, the N atom is sp2 hybridized (bond angle 120 not 107), and
the lone pair is delocalized by resonance into the benzene ring,
making it a strong activator and an ortho-para director. Hence 2 Br
atoms (from 2 moles of aqueous Br2) will be substituted onto the
(single available) ortho position, and para position, relative to
the N atom. The 3rd mole of Br2(aq) undergoes electrophilic
addition with the alkene group.
Q3. Ammonia is a weak nucleophile, and there are no viable leaving
groups (both OH- and CN- are more unstable compared to NH3), and
hence neither SN1 nor SN2 are possible.
Q4. (i) Both factors simultaneously, of course. Which outweighs
which? Depends on context. For alkyl halides, the differing C-X
bond strength is the more important factor that determines rate of
hydrolysis (ie. going down Group 7). Comparing alkyl halides vs
acyl halidies, then the much greater magnitude of partial positive
charge on the acyl C atom, predominates to reduce the Ea required,
allowing for instantaneous hydrolysis (at room temperature),
compared to hydrolysing alkyl halides where heating under reflux is
required.
(ii) Comparing hydrolysis rates of esters & amides versus acyl
halides :
This is beyond the syllabus, and not all JC teachers understand
this well, let alone JC students. So unlikely for Cambridge to ask
students to explain, but of course students need to be able to at
least state so.
There are 2 reasons for the differing rates of hydrolysis :
Resonance within the reactant, and stability of the intermediate
products.
As mentioned in my previous post, esters & amides are resonance
stabilized ; to be precise the C atom in the resonance hybrid of
esters & amides are not as electron deficient (and thus not as
electrophilic) compared to aldehydes, ketones and acyl halides. For
aldehydes and ketones, it's because there is no adjacent O and N or
halogen atom to donate electrons by resonance. For acyl halides,
the halogen atom can and does donate electrons by resonance, but
not as effectively compared to O or N atoms in esters & amides.
Why? Case 1 : F is the most electronegative element in the
Universe, so it withdraws electrons by induction even more strongly
than it donates by resonance. Case 2 : Cl, Br, I don't withdraw
much by induction, but they donate even more poorly by resonance
(ie. weak partial double bond character), due to the ineffective
sideways overlap between their diffused 3p or 4p or 5p orbitals
with the 2p orbital of the C atom. Consequently, the magnitude of
partial positive charge on the C atom in acyl halides is (all
things considered) still larger, compared to in esters & amides
(where the adjacent O or N atom donates electrons by resonance to
the C atom, counteracting the electron-withdrawing by resonance
effects of the acyl O atom).
Stability of the intermediate products : Notice that while acyl
halides can be readily hydrolysed under neutral pH aqueous
conditions at room temperature, hydrolysis require both an acidic
or alkaline pH, as well as heating under reflux. The higher
temperature due to the higher Ea is attributed to both the
resonance factor mentioned above, as well as the higher Ea required
for the elementary steps in the extended mechanism pathway for
hydrolysis of esters & amides (compared to the simple
addition-elimination mechanism for acyl halides). At neutral pH,
should the nucleophilic acyl substitution occur in a simplistic SN1
or SN2 or addition-elimination mechanism (which it does not),
notice that the leaving group, upon elimination has a negative
formal and hence ionic charge. Halide ions Cl-, Br- and I- are
relatively stable, due to their low anionic charge densities, due
to their large ionic radii. But a negative formal charge on a small
O or N atom (on the group eliminated during hydrolysis of esters
& amides) is strongly destabilizing, due to high anionic charge
density on the small O or N atom. Hence an acidic pH is required
for proton transfers to lower the Ea for the elimination elementary
step. An alkaline pH doesn't lower the Ea for this elementary step,
but it does lower the Ea for the preceding elementary step, as OH-
(during alkaline hydrolysis) is more electron-rich and thus a
stronger nucleophile (thus lower Ea) compared to H2O (during
neutral or acidic hydrolysis).
If you're interested, google out (and subsequent to perusing them,
draw them out yourself to explain to your students if they're
interested and ready) the full curved-arrow electron-flow
mechanisms for :
Hydrolysis of acyl halides
Acidic hydrolysis of ester
Alkaline hydrolysis of ester
Acidic hydrolysis of amide
Alkaline hydrolysis of amide
and compare the Ea required for each and all elementary steps, to
better understand why acyl halides undergo hydrolysis so much more
readily than esters & amides.
PS. btw & fyi, the terms AAC1, AAC2,
AAL1, AAL2, BAC1, BAC2,
BAL1, BAL2 used for describing the varying
acid-catalyzed vs base-promoted, acyl cleavage vs alkyl cleavage,
mechanisms for ester & amide hydrolyses, are *not* required and
go beyond even H3 Chem level. For H2 & H3 Chem, just regard the
hydrolyses mechanisms for esters & amides to be an *extended*
addition-elimination mechanism focusing on acyl (rather than alkyl)
cleavages, involving a tetrahedral sp3 intermediate, with proton
transfers in between.
PPS. btw & fyi, if Cambridge asks for the type of reaction
(rather than type of mechanism), hydrolysis of acyl halides, esters
& amides are all considered "nucleophilic acyl substitutions"
(just as SN1 and SN2 are mechanisms for nucleophilic aliphatic
substitutions). So give both "hydrolysis" and "nucleophilic acyl
substitution" as the answer. Type of mechanism for the hydrolyses,
will be "addition-elimination" (term will suffice for H2 & H3
Chem, but you'll need to draw out the full mechanism for H3 Chem),
or if you really feel like showing off, "AAC1,
AAC2, AAL1, AAL2, BAC1,
BAC2, BAL1, BAL2" for hydrolysis
of esters & amides. For generation of esters & amides, type
of reaction is still "nucleophilic acyl substitution" but now also
"condensation" instead of "hydrolysis". Type of mechanism is still
"addition-elimination".
PPPS. btw & fyi, Do your students know the definitional
difference between "condensation" vs "dehydration"? Or "hydrolysis"
vs "hydration"? (Note that for these 4 terms, there's somewhat
different usage between Organic Chem vs Physical Chem contexts).
Not many JC students know, and not many JC teachers bother to
teach, or teach correctly, on these matters. (Gohby and everyone
else reading this post who might be interested, I'll leave it to
you to go ahead and explore for yourself these terms, rather than
explicitly revealing them on the forum ; if you're a JC student you
can try asking your school teacher or private tutor on the
definitions and correct usage of these terms, in physical vs
organic chemistry contexts *evil laugh*).
