chem qn

last SG · 2011-11-16T17:55:17+00:00

Thanks taking A levels… formal charge is the overall charge right? Btw why does phenol form a trisubstituted product when br2(aq) is used but not when br2(c...

UltimaOnline SG

okay. That qn was reduction of ketone to alcohol so i guess nabh4 can be used. Smile
See if my explanation is right:
alh4- is easier to be polarised due to the greater size of electron cloud of al as compared to b. The hydride ions are nucleophiles as they are negatively charged. Are the electrophiles the compound that are going to be reduced? Is the mechanism similar to tat of nuc sub of RX? Btw, why is reduction of alkenes not electrophilic addition?

Yes, that's right. Whether it's CN- (from NaCN) or H- (from LiAlH4), or R- (from Grignard reagent), the mechanism is exactly the same : guys like girls, so nucleophiles like electrophiles, the nucleophiles shoot out their 'balls' (ie. lone pair) for attack (ie. donate a dative bond) to the electrophile (eg. ketone or aldehyde or carboxylic acid or ester) being reduced.

Alkenes to alkanes, is indeed electrophilic addition, but because the term "hydrogenation" is more precise and informative, you must give "hydrogenation" rather than "electrophilic addition" for converting an alkene to an alkane.

pokepokee SG

thanks. For the last part, i went to wiki and it said it is not electro add. Not sure if my reasoning is sound:
In hydrogenation, pt/ni as act a heterogenous catalyst, thus h2 and alkene molecules will be adsorb at the surface of the cat. In the case of electro addition, there is an unequal sharing of e, but hydrogen has only 1 electron, thus partial +ve/-ve charges cannot be formed. Also, there is no carbocation intermediate, thus hydrogenation is not electrophilic addition

UltimaOnline SG

You're confusing mechanism with reaction type. "Electrophilic addition" is not a mechanism.

The mechanism for hydrogenation of alkenes require a transition metal catalyst, therefore the mechanism is described as "catalytic hydrogenation".

But as long as alkene is the nucleophile (guy) here, and hydrogen (when interacting with the transition metal catalyst) behaves as the electrophile (girl) here, the reaction type is still considered "electrophilic addition", since you're adding the smaller electrophile (girl) to the larger nucleophile (guy).

As for your statement "since hydrogen has only 1 electron, thus partial +ve/-ve charges cannot be formed" shows you're confusing partial charges with formal charges and ionic charges.

UltimaOnline SG

thanks. For the last part, i went to wiki and it said it is not electro add.

You made one more glaring error.

Wikipedia says hydrogenation is an electrophilic addition. Look under "Typical electrophilic additions" :

http://en.wikipedia.org/wiki/Electrophilic_addition

pokepokee SG

okok. Chill, dont need to be so agitated. Smile I clarified this as my notes stated that hydrogenation is not electrophilic addition..

pokepokee SG

Hey. Some qns regarding tys mcq:
1) Is it right to assume that phenol does react with an acyl chloride to form an ester? I know that phenol is a weak nucleophile and it must be converted to a phenoxide ion. So the reaction btw phenol and acyl chloride proceeds at a slower rate than phenoxide ion and acyl chloride?
2)07/p1 qn 3 An excess of cold water was added to 0.3mol of a chloride in third period. 0.6molof hcl was formed. What was the chloride? Ans is pcl5.i thought pcl5 reacts in limited water to form pocl3 and 2hcl? Why cant the ans be mgcl2?

UltimaOnline SG

Q1.
Acyl halides are indeed so feminine & chio (ie. electrophilic) that even guys (ie. nucleophiles) with weak balls (ie. not so electron-rich) are able to sexually (ie. chemically) react with. Deprotonating phenol to generate the stronger nucleophile phenoxide ion (ie. O atom with a -ve formal charge instead of merely a -ve partial charge) may in theory lower Ea and increase rate of reaction, but you've to be careful not to use excess NaOH (and it must be anhydrous!), else you'll be wasting your precious acyl halide chiobu that will be brutally ravaged away by your hired OH- ion mercenaries / soldiers / gangsters, as well as the horny water molecules if you had used aqueous NaOH.

Q2.
The charge density of Mg2+ is not sufficiently high to generate 2x the moles of HCl. It only undergoes partial hydrolysis to generate a small fraction (relative to the moles of MgCl2) of HCl.

Temperature is actually more important than limited/excess water. (Just as temperature is the more important factor when determining oxidation of alkenes to diols versus oxidative cleavage, over other factors such as pH and concentration).

Hence in cold water, 1 mole of PCl5 reacts with 1 mole of H2O to generate 1 mole of POCl3 and 2 moles of HCl.

pokepokee SG

Ok. thanks Smile
1) So concentrated naoh should be used to generate the phenoxide ion?
2) Can the explanation be used to explain why the ph of mgcl2 in water is 6.5 instead of 1?
3) I read cs toh’s guide book and there is a phrase ”__ more susceptible to electrophilic attack” I thought nucleophiles are the horny guys that will attck, not the electrophiles?

UltimaOnline SG

Q1. When reacting with acyl halides, phenol is sufficient. You only need to deprotonate phenol if you want to react with less feminine (electrophilic) species such as alkyl halides (aka halogenoalkanes).

Q2. Yes. Al3+ is more acidic compared to Mg2+, because Al3+ has a significantly higher charge density, which means that it is more strongly electron-withdrawing by induction, and withdraws electron density more strongly from the water ligands, weakening the OH bonds of the water ligands, allowing protons to dissociate more readily (as the proton dissociation enthalpy becomes less endothermic).

Q3. In terms of mechanism, only nucleophiles (guys) have the balls to attack electrophiles (girls), 100% of the time, no exception (whether it is nucleophilic addition or electrophilic addition, it is still the nucleophile attacking the electrophile). The curved (or curly) arrows in a mechanism, illustrates the nucleophiles shooting out its balls (either a lone pair for OH- CN- Cl- nucleophiles, or pi bond for alkene and benzene nucleophiles) to form a new dative bond with the electrophile. In terms of wording, you can exercise a degree of flexibility and use both terms "nucleophilic attack" and "electrophilic attack", the latter term (as used by CS Toh) really meaning "reacting with the electrophile", or "the electrophile kena attack by".

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