Originally posted by theophilus:Thanks !!
Also for practical what do they provide ah? like do they pass give you the QA results data booklet all ?
AJC 2013 P3
qn 1aiv)
NAD+ is reduced to NADH, while ethanol is oxidised to ethanoate ions
shouldnt deltaG be -ve150kjmol-1 instead of +150kjmol-1 ?
as +150kjmol-1 calculated for delta G is for Ethanoate ions to Ethanol?
Originally posted by Flying grenade:AJC 2013 P3
qn 1aiv)
NAD+ is reduced to NADH, while ethanol is oxidised to ethanoate ions
shouldnt deltaG be -ve150kjmol-1 instead of +150kjmol-1 ?
as +150kjmol-1 calculated for delta G is for Ethanoate ions to Ethanol?
isn't what's happening is that,
NAD+ is reduced to NADH, while ethanol is oxidised to ethanoate ions ---(1) ?
and qn asks to calculate deltaG for the above redox rxn?
+150kjmol-1 calculated for delta G is for Ethanoate ions to Ethanol
so wouldn't delta G for (1) be -150kjmol-1?
Originally posted by Flying grenade:isn't what's happening is that,
NAD+ is reduced to NADH, while ethanol is oxidised to ethanoate ions ---(1) ?
and qn asks to calculate deltaG for the above redox rxn?
+150kjmol-1 calculated for delta G is for Ethanoate ions to Ethanol
so wouldn't delta G for (1) be -150kjmol-1?
Alright , Thank you !
Hi, for JJC 2014 prelim P2 Q4a iv,
Why can't the equations be Co/Co2+(-0.28V) and O2/H+(1.23V)? We can still obtain a non feasible reaction of -1.51V with these equations.
AJC 2013 qn 2biv
pic of qn here
https://www.dropbox.com/s/fgajn95r85n5qn2/20161024_141215_1-1.jpg?dl=0
i thought steric hindrance is often applied for groups directly bonded to the atom of interest(in this case the carbonyl carbon) ?
the bulky R groups connected to the carbon adjacent to the carbonyl carbon can also provide steric hindrance ?
Also it's the C atom connected to the carbonyl carbon in the ester( R-COO-R) that's changing, not the carbonyl carbon that's changing. the number of R groups connected to the adjacent carbon can also cause the carbonyl carbon to be less e- deficient? The more frequent cases we encounter is , the no. of R groups directly connected to the C atom of interest, or ethyl or methyl group
(ok i do realise that benzene ring or long alkyl chains do provide steric hindrance for a molecule, overall)
https://en.m.wikipedia.org/wiki/Oxytocin
the glycine constitutent amino acid in Oxytocin has its COOH replaced with CONH2 by nature/fact
Originally posted by supercat:Hi, for JJC 2014 prelim P2 Q4a iv,
Why can't the equations be Co/Co2+(-0.28V) and O2/H+(1.23V)? We can still obtain a non feasible reaction of -1.51V with these equations.
Originally posted by Flying grenade:AJC 2013 qn 2biv
pic of qn here
https://www.dropbox.com/s/fgajn95r85n5qn2/20161024_141215_1-1.jpg?dl=0
i thought steric hindrance is often applied for groups directly bonded to the atom of interest(in this case the carbonyl carbon) ?
the bulky R groups connected to the carbon adjacent to the carbonyl carbon can also provide steric hindrance ?
Also it's the C atom connected to the carbonyl carbon in the ester( R-COO-R) that's changing, not the carbonyl carbon that's changing. the number of R groups connected to the adjacent carbon can also cause the carbonyl carbon to be less e- deficient? The more frequent cases we encounter is , the no. of R groups directly connected to the C atom of interest, or ethyl or methyl group
(ok i do realise that benzene ring or long alkyl chains do provide steric hindrance for a molecule, overall)
Inductive effects fall rapidly with distance. Hence in this question, only steric hindrance is a correct reason, the inductive effect is insignificant. So fair enough, you're somewhat correct in your thinking here.
does the Magnitude of pka signify whether a Group is more acidic than another group? it doesn't signify is it?
alpha COOH group is always more acidic than Rgroup *acidic* group right? i.e. no need to be Rgroup COOH?
please refer to BFJC Isolectric points worksheet, idw take photo because mine is very messy
e.g. For Tyrosine, alpha NH3+ group is less acidic than Rgroupacidic group, but pka is 9.2 which is less than 10.5(pka acidic R group)
for Histidine,
Rgroup Basic group(which is more basic) has smaller pka , which in turn has bigger pkb value , than alpha Basic group
smaller pka and pkb, more acidic and basic respectively
i know how to calculate pI if given the pka values for all the groups.
i just want to ask pI = (pka1+pka2)/2 is not true for all amino acids right? need ur confirmation so i can cross it out on my paper and my head
but magnitude of ka(and kb) and pka(and pkb )does signify acidity. larger ka, more acidic, smaller ka, less acidic
like kb of ammonia = 1.8x10^-15
ka of ethanoic acid = 1.8x10^-15
One way to create . no wrong
The only way to create a zwitterion with 3 groups(not 4) is to protonate the most basic group and deprotonate the most acidic group while ensuring the 3rd group remains neutral. (cannot deprotonate a group less acidic than one more acidic first, nor protonate a group less basic than one more basic first)
pka alphaNH3+ of Tyrosine = 9.2,
pka acidic R group of Tyrosine = 10.5
why is it that acidic R group has a higher pka despite being more acidic than alpha NH3+
got it for AJC p1 2013 qn 35
AJC 2013 p1 qn 36.
since Huckel's rule is not correct 100% of the time, Hückel's rule is not valid for many compounds containing more than three fused aromatic nuclei in a cyclic fashion. https://en.m.wikipedia.org/wiki/Hückel%27s_rule
how does molecule 2 exhibit aromaticity in reality ??
why doesnt https://en.m.wikipedia.org/wiki/Cyclooctatetraene exhibit aromaticity? oh cool ! look at the structure and bonding . one of the reason is that the molecule is not planar hence does not exhibit aromaticity
https://en.m.wikipedia.org/wiki/Cyclooctatetraenide_anion. this does
My apologies. Sorry.
let me repost the qns, deleting very taxing
Qn : pka alphaNH3+ of Tyrosine = 9.2,
pka acidic R group of Tyrosine = 10.5
why is it that acidic R group has a higher pka despite being more acidic than alpha NH3+ group
short and sweet
for Histidine,
why Rgroup Basic group(which is more basic) has smaller pka , which in turn has bigger pkb value , than alpha Basic group ?
Originally posted by Flying grenade:My apologies. Sorry.
let me repost the qns, deleting very taxing
Qn : pka alphaNH3+ of Tyrosine = 9.2,
pka acidic R group of Tyrosine = 10.5
why is it that acidic R group has a higher pka despite being more acidic than alpha NH3+ group
short and sweet
Originally posted by Flying grenade:for Histidine,
why Rgroup Basic group(which is more basic) has smaller pka , which in turn has bigger pkb value , than alpha Basic group ?
The R group iminium is less acidic (ie. smaller Ka or larger pKa) than the alpha COOH, but more acidic (ie. larger Ka or smaller pKa) than the alpha NH3+.
The R group imine is more basic (ie. larger Kb or smaller pKb) than the alpha COO-, but less basic (ie. smaller Kb or larger pKb) than the alpha NH2.
Bonus : For a challenging A grade question, Cambridge can ask you to *explain* (ie. not just assign values without explanation) why is it the case that for Histidine (unusual among alpha amino acids), the R group imine is more basic (ie. larger Kb or smaller pKb) than both the alpha COO- and the R group amine, and why the R group imine is less basic (ie. smaller Kb or larger pKb) than the alpha amine. Or conversely, explain the relative acidities of the conjugate acids of these (abovementioned) groups and their Ka or pKa values.
My BedokFunland JC students can check their answers with me during their next tuition session, while the rest of you can go ask your school teacher or your own private tutor. I won't reveal the answers here.
pI = (pka1+pka2)/2 is not true for all amino acids right?
this formula/rule of thumb no good right?
this was a qn i posted on top,
so for the hypothetical amino acid, with 4 groups,
the Rgroup*COOH* being always more acidic than
alpha NH3+
only works for *RgroupCooh* ,not Rgroup phenol and any other kind of R group?
all my doubt pertaining to this topic originates from this
ka(alpha cooh) > Rgroup(COOH) > AlphaNH3+ > Rgroup NH3+
kb (Rgroup NH2) > alphaNH2 > Rgroup COO- > Alpha COO-
so this is not always true? only for NH3 and COOH and not for other types of groups is it?
Originally posted by Flying grenade:this was a qn i posted on top,
so for the hypothetical amino acid, with 4 groups,
the Rgroup*COOH* being always more acidic than
alpha NH3+
only works for *RgroupCooh* ,not Rgroup phenol and any other kind of R group?
Never apply any formulae blindly (at least for H2 Chem, and at least if you're an intelligent student who seeks to understand rather than blindly memorize). If you understand why a formula works, then you will know how to correctly apply it across different contexts, or even tweak it when necessary.