LABILE PROTONS [Hs]

hoay SG

The peak of an alcohol disapperars when D2O is added. Do the peaks of COOH and NH2 and amide CONH2 also disappears on adding D2O?? If not then please explain why. Which protons are termed as labile??

Thank you.

UltimaOnline SG

The peak of an alcohol disapperars when D2O is added. Do the peaks of COOH and NH2 and amide CONH2 also disappears on adding D2O?? If not then please explain why. Which protons are termed as labile?? Thank you.

Hohoho.

Analytical Chem (including all spectroscopic methods) have long been taken out of the standard (ie. H2) Singapore A level syllabus. As such, I've quite deliberately forgotten all the details of the various spectroscopies.

If anyone else (eg. ChemGuide) would like to assist Hoay, please go ahead.

If no one replies, you probably want to post your questions on Chemical Forums.

hoay SG

Ok sir. One more query.

Nov 2006 P2 Q.3(d)

The bromine atom in CH2=CHCH2Br is very reactive.

The bromine atom in CH2=CHBr is unreactive.

Suggest an explanation for the unreactivity of the bromine atom in CH2=CHBr

Ans. One Br is connected to Carbon having single bond while the other is connected to a double-boned carbon so more susceptible to nucleophilic attack by alkalis or nucleophiles because it is elecrtophilic. While in the Br-connected to double bond is less prone to attack since C=C are more raective than the C-Br bond and secondly the nuclephile will be repelled by the incoming nucleophile. Is it the right appproach ??

UltimaOnline SG

Ok sir. One more query.

Nov 2006 P2 Q.3(d)

The bromine atom in CH2=CHCH2Br is very reactive.

The bromine atom in CH2=CHBr is unreactive.

Suggest an explanation for the unreactivity of the bromine atom in CH2=CHBr

Yes, you've got one reason right : for CH2=CHBr, the incoming nucleophile (eg. OH-) will be repelled by the nearby (ie. close proximity) electron-rich nucleophilic alkene double bond functional group, which thus significantly increases the Ea required for nucleophilic substitutions.

The other reason is :
Due to the sideways overlap of the p orbital of the Br atom and the pi orbital of the alkene functional group in CH2=CHBr, a lone pair on the Br atom is delocalized by resonance to form a pi bond with the sp2 hybridized C atom of the alkene, resulting in the C-Br bond having partial double bond character (ie. more endothermic bond dissociation enthalpy) in the resonance hybrid, which thus significantly increases the Ea required for nucleophilic substitutions.

Chemguide7 SG

For 1st question, you may wanna see:

http://books.google.com.sg/books?id=QWq47QClOM0C&pg=PA316&lpg=PA316&dq=amine+protons+labile&source=bl&ots=kI5Zdud4a8&sig=6puNuCNrhPuGyQmioQpieO-X7oQ&hl=en&sa=X&ei=ephlUaOSEqyhiAfAp4CwCQ&ved=0CGUQ6AEwCA#v=onepage&q=amine%20protons%20labile&f=false

Yes, the proton in -COOH can exchange itself rapidly with D2O solvent to form -COOD. Amines and amides protons can do so with D2O solvent under some conditions.

I suppose you are talking about 1H NMR spectra.

Wow! I did this 10-17 years ago. I can really play on Youtube now the song "Memory". *chuckle*

hoay SG

Generally speaking increasing the concentration of a reactant incraesing the yield of a product. But if the yield of the product increases will the value of the Kc increases because the reaction moves to formward direction?? If not why??

What can change the value of Kc??

UltimaOnline SG

Only changing temperature will change the Kc value. Changing any other variable (eg. moles or molarity or partial pressure of a reactant or product) will change the Qc value.

So even if we say position of equilibrium has shifted, it doesn't necessarily mean the Kc value changed. It could be that Kc value has changed (if we changed temperature), or it could be that Qc value has changed (if we changed the moles or molarity or partial pressure of a reactant or a product).

Either way, when changing either the Qc value or the Kc value, we say the position of equilibrium (ie. Kc) will have shifted, relative to Qc.

If Qc is now less than Kc, the position of equilibrium now lies to the right, and the rate of the forward reaction will exceed the rate of the backward reaction, until Qc = Kc and equilibrium is re-estabilished.

If Qc is now more than Kc, the position of equilibrium now lies to the left, and the rate of the backward reaction will exceed the the rate of the forward reaction, until Qc = Kc and equilibrium is re-estabilished.

hoay SG

Standard electrode potential is the emf measured between a test electrode and S.H.E.

under S.T.P. Standard redox potential is same as standard electrode potential??

How will we define standard cell potential in words??

UltimaOnline SG

Standard electrode potential is the emf measured between a test electrode and S.H.E.

under S.T.P. Standard redox potential is same as standard electrode potential??

How will we define standard cell potential in words??

There is no standard (pun intended) word definition for cell potential.

For A level purposes, Cambridge only requires students to define electrode potential in words (as you've done), and cell potential by formula, ie.

Cell Potential = Reduction Potential at Cathode + Oxidation Potential at Anode.
or
Cell Potential = Reduction Potential at Cathode - Reduction Potential at Anode.

Regarding RTP vs STP, the Data Booklet values are for standard conditions (of 1atm or 1bar for all gases, and 1 mol/dm3 for all solutions), and for temperature at 25 deg C.

In other words, the term "standard conditions" does not say anything about the temperature, which has to be specified separately. Teachers who teach that "standard conditions" in this context refers to 25 deg C, are teaching their students incorrectly. "Standard conditions" imply 25 deg C only for other Chemistry topics such as Energetics, but not for Electrochemistry.

hoay SG

Both Standard electrode potential and standard cell potential are same thing since the equation that you gave above regarding Cell potential will still give the standard electrode potential of any cell in consideration. Is that so?

UltimaOnline SG

Not quite.

Technically, the term Standard Electrode Potential refers to either the Oxidation Potential or (more usually, by convention) the Reduction Potential, for a particular Electrode (which could be either the Cathode or the Anode, relative to the SHE), under Standard conditions.

In contrast, the term Standard Cell Potential compulsorily takes into consideration both Redox Potentials, again under Standard conditions, of both Electrodes (ie. both the Cathode and the Anode) that constitute the ElectroChemical (usually Galvanic aka Voltaic) Cell.

Perhaps the source of many students' (and some school teachers' as well) confusion, is that the Standard Electrode Potential = Standard Cell Potential, if and only if, one of the Electrodes in the Cell, happens to be (by deliberate choice) the Standard Hydrogen Electrode (SHE).

Because the SHE's Electrode Potential (either/both Reduction and/or Oxidation Potentials) is designated by humans to be taken as the default zero Volts (ie. 0 V), hence the Standard Cell Potential for such a setup (and only such a setup) = the Electrode Potential for the Electrode being investigated.

See Wikipedia :
http://en.wikipedia.org/wiki/Electrode_potential

hoay SG

The line of best fits is the line on a graph that covers all the points. What is the "curve of best fit"?

UltimaOnline SG

The line of best fits is the line on a graph that covers all the points. What is the "curve of best fit"?

It is a smooth curve (either hand-drawn or graphic calculator generated) that best fits most of the points plotted. Similar to a straight line of best fit, there will be outliers for a curve of best fit. These outliers may be due to experimental error, etc.