2016 H2 Chemistry JC1 & 2 students post your questions here
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From the number of views my H2 Chemistry threads on this forum has garnered, it's pretty clear that hundreds of lurkers (presumably and hopefully the majority of whom are actual JC students) are interested in my H2 Chemistry posts.
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Just register an account with SgForums, it's easy and free (of course, asking for free online help here... or for that matter anywhere... isn't as effective as actually joining my BedokFunland JC tuition, but it's still better than having no one reliable to ask for help... or if you disagree with your own private tutor or school teacher and wish to have a 2nd opinion that's reliable and trustworthy) to ask anything relevant to H2 Chemistry here.
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What are alcohols? What are the drinkable alcohols? Why are some alcohols not drinkable?
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Why do we look at strength of conjugate base when determining acid strength?
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Compare and contrast the reactions of ethanol and phenol with each of the following reagents:
a) ethanoic acid
b) potassium manganate (VII)
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Why is CH3CH2CH(OH)2 unstable?
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Why is the product of SN2 inverted from that of the starting organic molecule?
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ClCH2COCL + NH2CH2CH2OH -> E -> add Na and warm -> F What is E and F?
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Originally posted by Loh.huiyuen:
What are alcohols? What are the drinkable alcohols? Why are some alcohols not drinkable?
Only ethanol is drinkable (and even ethanol is considered slightly toxic). All other alcohols are a lot more toxic to humans, with the most common culprits of alcohol poisoning (due to the fact that they are readily available in non-consumable household or industrial products and subsequently used unethically and illegally as cheap and readily available replacements for ethanol to sell as drinkable alcohol to unsuspecting victims) being methanol and propan-2-ol. Specific technical details on why the different alcohols are toxic and their biochemical mechanisms in the human body require a biochemical and medical background to understand.
https://en.wikipedia.org/wiki/Alcohol#Toxicity
https://en.wikipedia.org/wiki/Methanol#Toxicity
https://en.wikipedia.org/wiki/Isopropyl_alcohol#Toxicology -
Originally posted by Ephemeral:
Why do we look at strength of conjugate base when determining acid strength?
To be precise, it's recommended (for A level exams) that you analyze the *stability* of the conjugate base, rather than the strength of the conjugate base. Of course, they're related insofar as the more stable the conjugate base (or any base), the weaker it is as a base.If after buying that million dollar Ferrari, you don't have any money left to eat, obviously you wouldn't buy it in the first place. If after buying that million dollar Ferrari, you still have lots of money left, then naturally you wouldn't mind buying the Ferrari in the first place, coz you can afford to buy it.
If after donating a H+, the conjugate base is highly unstable, then it wouldn't be willing to donate the H+ in the first place, and therefore the more unstable the conjugate base, the weaker the conjugate acid.
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Originally posted by Ng.keebin:
Compare and contrast the reactions of ethanol and phenol with each of the following reagents:
a) ethanoic acid
b) potassium manganate (VII)
Phenol is too weak a nucleophile (due to resonance delocalization of the lone pair) to attack the weak electrophile ethanoic acid, hence esterification does not occur, as it would with ethanol.
KMnO4 is able to oxidize ethanol, but not phenol, due to the absence of an alpha H atom that has to be eliminated during oxidation (when you eliminate a less electronegative atom, the bonded atom's oxidation state is forced to become more positive).
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Originally posted by nicolemantou:
Why is CH3CH2CH(OH)2 unstable?
Because it is a geminal diol, whose close proximity of the 2 OH groups allow elimination of H2O to occur with thermodynamically favourable positive entropy change.
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Originally posted by nicolemantou:
Why is the product of SN2 inverted from that of the starting organic molecule?
Animation for SN2 inversion of configuration :
https://www.youtube.com/watch?v=o7hnYlfwMMM
Source : https://en.wikipedia.org/wiki/Walden_inversion -
Originally posted by nicolemantou:
ClCH2COCL + NH2CH2CH2OH -> E -> add Na and warm -> F
What is E and F?
First identify which is the stronger nucleophilic group (amine vs alcohol) and which is the more reactive electrophilic group (alkyl halide vs acyl halide). Cambridge may ask you to explain their relative reactivities.
Hence, nucleophilic acyl substitution first occurs to generate amide (instead of ester). Next, Na deprotonates OH, the subsequent alkoxide conjugate base nucleophilically attacks the alkyl halide via SN2 (both intramolecularly and intermolecularly are possible, especially when such a large ring involves less ring strain due to angle strain).
If the question doesn't provide additional data or conditions in regards to the product, the exam-smart student will give both answers in the A level exams, with labels as to which is the intramolecular SN2 product and which is the intermolecular SN2 product. -
Why did you name yourself UltimaOnline?
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Originally posted by Loh.huiyuen:
Why did you name yourself UltimaOnline?
Because for over a decade I used to be part of the granddaddy of all modern day MMORPGs : Ultima Online. -
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https://twitter.com/_hykel/status/693749339173339136
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Originally posted by hykel:
https://twitter.com/_hykel/status/693749339173339136
a) look up hydrogenation.
b) either KMnO4 or K2Cr2O7
c) either PCl5 or SOCl2 positive test for alcohols, or 2,4-DNPH positive test for ketones. -
https://www.facebook.com/photo.php?fbid=10204073499076458&set=a.1524464970271.61590.1794053279&type=3
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https://twitter.com/itsliyingg/status/693752544061829121
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Originally posted by Liying98:
https://twitter.com/itsliyingg/status/693752544061829121
D is CH3CH2COCH3, while E and F are enantiomers of CH3CH2CH(OH)CH3. -
https://twitter.com/itsliyingg/status/693776550546673664
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Originally posted by Liying98:
https://twitter.com/itsliyingg/status/693776550546673664
phenol ; aldehyde or ketone ; COOH group since C contains 4 O atoms (and 2 O atoms are accounted for by aldehyde/ketone and phenol) ; based on the difference in molar masses between C and product of rxn 2, you can deduce that 2 H atoms have been substituted by 2 Br atoms ; PCl5 first converts COOH group to COCl electrophilic group, which is subsequently attacked by the nucleophilic phenolic group in an intramolecular nucleophilic acyl substitution (mechanism is addition-elimination) to generate the cyclic ring, and you know the ketone group must be between the benzene ring and the COOH group, so as to prevent excessive ring strain due to angle strain and allow the cyclic ring to form in the condensation, addition-elimination, nucleophilic acyl substitution reaction. -
Why does pKa value of hydrohalic acids decrease down the group?
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Salicyclic acid contains both a hydroxyl and a carboxylic acid group. Why will the hydroxyl group bonded directly to the benzene ring not be substituted upon addition of SOCl2?