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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

UltimaOnline SG

Can tyrosine's phenol group interact with nh3+?

If cannot, is it because phenol doesn't ionise in water?

Hydrogen bonding is possible, between the partial positive H atom of NH3+ (the H bond donor), and the partial negative O atom of phenol (the H bond acceptor).

Cambridge already stated in the question, regarding extent of deprotonation of phenol at pH 7. Read the question carefully lah!

Flying grenade SG

Wtf sia, then why we learn polypeptide linkages and proteins undergo hydrolysis and break into smaller portions?

And thought ph7 the acid and basic groups ionise?

UltimaOnline SG

You need to either heat under reflux or use biological enzymes, plus at strongly acidic or strongly basic pH lah. That's how your strongly acidic stomach digests the meat you eat, with proteases enzymes.

Acidic COOH groups and basic NH2 groups do ionize via hydrolysis to generate COO- groups and NH3+ groups.

Different from hydrolysis of the protein's peptide bond within the amide group in the polypeptide chain of a protein (the word "peptide" refers to *either* the peptide bond within the amide group in a polypeptide chain of a protein, *or* the amino acid residues in a polypeptide chain of a protein). Because the amide group within a polypeptide chain bonds two amino acid residues or 'peptides' together, the amide group in proteins is called the peptide group, and the C-N bond within the amide group is called the peptide bond, linking two 'peptides' or amino acid residues together.

Hydrolysis means "chemical reaction with water" (where chemical reactions involve breaking or forming of sigma bonds, while resonance involves breaking or forming of pi bonds only), while hydration means "physical interaction with water" (physical, inorganic & organic chemistry) or "addition of water" (in some organic chemistry contexts, eg. hydration of alkenes to alcohols).

Flying grenade SG

2013 paper 1 qn 27

Are we supposed to know if pcl5 reacts with oh or cooh group?

I only know pcl5 is a acidic chloride and it may react with oh group

UltimaOnline SG

2013 paper 1 qn 27

Are we supposed to know if pcl5 reacts with oh or cooh group?

I only know pcl5 is a acidic chloride and it may react with oh group

-_-" Have you even memorized your CS Toh A level Study guide? You *must* memorize the entire book, if you expect to even *pass* your A levels (coming up in the next few days). Instead of spoonfeeding you (basic stuff like these), you go ahead and look up (in CS Toh book, or google or Wikipedia) PCl5 and tell me what it reacts with.

Flying grenade SG

Pcl5 reacts with acid to form acyl chloride

Reacts with alcohol to from halogenoalkane

^both via nucleophilic substitution

UltimaOnline SG

Pcl5 reacts with acid to form acyl chloride

Reacts with alcohol to from halogenoalkane

^both via nucleophilic substitution

Correct.

PCl5 (and for that matter, SOCl2 as well) doesn't react with phenol to generate aryl halides though, for 2 reasons.

1stly, the unlike aliphatic alcohols, the lone pair on the phenolic sp2 O atom is delocalized by resonance to form a pi bond with the sp2 C atom of the benzene ring in some resonance contributors), exactly as is the case with aryl and vinyl halides, and thus the resonance hybrid's phenolic O atom is electron-deficient (has a positive formal charge on some contributors, and hence a partial positive charge in the resonance hybrid), and thus is non-nucleophilic and cannot attack the electrophilic P atom of PCl5 (or the electrophilic S atom of SOCl2).

2ndly (even for the small % of phenols which happen to possess sufficient activation energy to attack PCl5 or SOCl2), the electron-rich Cl- nucleophile (eliminated from PCl5 or SOCl2 after reacting with ROH) is electrostatically repelled by the pi electrons of the electron-rich benzene ring nucleophile, and thus PCl5 and SOCl2 cannot complete the mechanism to convert phenol to aryl halides (which therefore does not happen).

If descriptions of the elementary steps are provided in the question, Cambridge could ask you (eg. in this year's A level exam) to draw the (partial or full) mechanism for PX5 or PX3 or SOX2 reacting with alcohols and carboxylic acids to generate alkyl halides and acyl halides respectively. So for those of you interested (and ready ; if any of these confuses you, then you're not ready and you should skip this entirely), here are the mechanisms

(Bonus teaser challenge to all JC students : the use of SOCl2 is often favoured over PCl5, because the SOCl2 reaction has an important advantage over the PCl5 reaction, can you guess what it is? Cambridge could ask this question sooner or later, and I won't reveal the answer, up to those aiming for a distinction to figure it out for yourselves.)

Mechanism for SOCl2 and PBr3 reacting with alcohols to generate alkyl chloride and alkyl bromide :
https://www.khanacademy.org/science/organic-chemistry/alcohols-ethers-epoxides-sulfides/reactions-alcohols-tutorial/v/preparation-of-alkyl-halides-from-alcohols

Mechanism for SOCl2 reacting with carboxylic acids to generate acyl chloride :
https://www.khanacademy.org/science/organic-chemistry/carboxylic-acids-derivatives/reactions-carboxylic-jay/v/preparation-of-acyl-acid-chlorides

Mechanism for PCl5 reacting with carboxylic acids to generate acyl chloride :
https://upload.wikimedia.org/wikipedia/commons/7/71/Phosphorus_pentachloride_mechanism.png

Note that the phosphoryl chloride generated can be further hydrolyzed, this is covered in the H2 syllabus under the inorganic chemistry topic : periodicity. In fact, the mechanism for both water H2O (inorganic chemistry) and alcohol ROH (organic chemistry) reacting with PCl5, is exactly the same (up to a point), because did you notice that water (H-O-H) is a 'type' of alcohol (R-O-H) in which the R group is a H atom, and thus it should make natural and logical sense to you (did your school teach this to you?) that the reaction mechanism (up to a point) is exactly the same : HOH with PCl5 gives POCl3 and 2HCl, while ROH with PCl5 gives POCl3 and RCl + HCl.

You can try drawing the mechanism for further hydrolysis of POCl3 to give H3PO4 + 3HCl (notice that the OS of all elements remain unchanged during hydrolysis reactions, so based on the starting OS, you can deduce the final OS which will help you deduce the structure or formula of the final hydrolysis product). The mechanism involves 3 water molecule nucleophiles (one by one) attacking the electrophilic P atom of POCl3, thereby eliminating the 3 Cl- ion leaving groups one by one, followed by proton transfers to generate the final products of H3PO4 and 3 HCl. Cambridge could ask you to draw out this fairly simple mechanism (of which the balanced equation you have to memorize under the Periodicity topic anyway, so might as well you draw out the mechanism to make sense of it for yourself instead of blindly memorizing the equation like most JC students do... which is just sad. Learning should always be fun and meaningful.)

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Btw, the following is something extra, a bonus for those of you (eg. Flying Grenade) who may be interested in studying Chemistry at the Uni level. Did you know that other than SN1 (racemization) and SN2 (inversion of configuration), there's a 3rd mechanism called SNi (retention of configuration) whose secret existence, bewildered chemists figured out, when trying to figure out why some reactions like SOCl2 with alcohols mysteriously resulted in alkyl chlorides with retention of configuration instead of the expected racemization or inversion of configuration (ie. meaning it was neither SN1 nor SN2). If hearing this excites you, then yup, you should certainly consider studying Chemistry at Uni level. Enjoy! :)

SOCl2 and the SNi Mechanism
http://www.masterorganicchemistry.com/2014/02/10/socl2-and-the-sni-mechanism/

Flying grenade SG

^ thank you ultima, for everything.

Socl2 thionyl chloride might be preferably used than pcl5 or pcl3 for the Nu substn for the conversion of alcohol to haloalkane is because the other products formed are gases: so2 and hcl that would escape from the soln, hence haloalkane is easily obtained,

Rather than an additional step of fractional distillation needed if used phosphorus chloride

But hcl and so2 are also poisonous, might nt be advantage in all occasions?

Flying grenade SG

Btw what/how do you think about the 'thomas bond, chris hughes A level books, by the themis yellowreef publisher haha

Flying grenade SG

Is there any significance of pyridine in H2 chem that we need to know?

UltimaOnline SG

^ thank you ultima, for everything.

Rather than an additional step of fractional distillation needed if used phosphorus chloride

But hcl and so2 are also poisonous, might nt be advantage in all occasions?

Correct, but that's not all. Think Le Chatelier's principle and Gibbs free energy (which is enthalpy + ???).

UltimaOnline SG

Btw what/how do you think about the 'thomas bond, chris hughes A level books, by the themis yellowreef publisher haha

Do you see it in my BedokFunland JC's List of Recommended Books? Nuff said. ;Ă

UltimaOnline SG

Is there any significance of pyridine in H2 chem that we need to know?

Nope, but of course Cambridge can show you the structure and ask you qns on it. For instance, "Is pyridine likely to be a stronger nucleophile or base?" or "Compare pyridine's basicity with that of phenylamine", etc.

Flying grenade SG

For the Reaction with socl2

đşS increases cos formation of 2 mol of gaseous product compared to 1 gaseous reactant

Gaseous products have more ways to arrange the moledcules and their energies ,hence entropy increases

Can help improve in explanation?

Err đşG is -ve becos reaction occur?

đźH is (-ve) exo???

Flying grenade SG

How to use le Chatelier's Principle to explain the advantage and or why it is favored?

(Bonus teaser challenge to all JC students : the use of SOCl2 is often favoured over PCl5, because the SOCl2 reaction has an important advantage over the PCl5 reaction, can you guess what it is?

UltimaOnline SG

Don't just compare LHS with RHS for the SOCl2 reaction, but also the PCl5 reaction. In other words, the SOCl2 reaction generates 2 gaseous products, while PC5 reaction only generates 1 gaseous product.

Hence, the Gibbs free energy is expected to be more negative (ie. thermodynamically feasible) for the SOCl2 reaction compared to the PCl5 reaction, due to thermodynamically favourable positive entropy change.

Since SO2 is a gaseous product, which readily leaves the solvated reaction mixture, hence the position of equilibrium is pulled over (or if you prefer, shifts more) to the RHS, as predicted by Le Chatelier's principle.

Flying grenade SG

Regarding to a old qn, this just happen to come across my mind

Potassium-19 : [Ar] 4s1

Kr-34: [Ar] 3d10 4s2 4p6

Will the higher energy level of 4p6 e- ( and hence further distance from the nucleus)

Compounded by the additional shielding effect by 3d and 4s e- experienced 4p6 e-

Affect the atomic radius?

Is shieding effect provided from orbitals insignificant, only shielding effect provided by principal quantum electron shell significant?

Flying grenade SG

Oh so since eqm shifted to the right more, higher yield (compared to rxn with pcl5) of the products( halo alkane) is produced , hence its more favourable?

Flying grenade SG

Update :

i also thought of another factor

In period 3, the 3s1 e- in Na experience 11 charges of proton attractions, 3p6 e- on argon atom experiences 18 charges of proton attractions, a difference of 7 charges

In period 4, the 4s1 e- in K experiences 19 proton charges, 4p6 e- in Kr experiences 36 proton charges, a difference of 17 charges

So perhaps this might be a contributing factor as well?

Flying grenade SG

Do we need or when do we say this :

Gaseous products have more ways to arrange the molecules and their energies ,hence entropy increases

Can help improve explanation

Or just say thermodynamically feasible positive entropy change occurs

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