The Periodic Table - 2016-09-25 (page 1 of 2)

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3:01 AM

o/

Hullo!

3:30 AM

-1

Q: General chemistry II

I deleted my post cause I was told that my question was from homework. Well, yes that is true. If I knew how to solve, I wouldn't have come here. LOL

homework solubility

Wtf is wrong with this person..

3:55 AM

@Jan I'd just recommend asking a new question yourself... Tell us if you want the source code/screenshot of original question. @Mith

Morning!

> Microtitre plates can be centrifuged in Eppendorf 5810 R with swing out rotor A-4-62 and microplate holders.

Is this proper usage, in terms of language, or should one use an article, like "in an Eppendorf 5810 R unit"?

@CowperKettle I like your formulation better.

@orthocresol Really? I should ask on ELL SE then

@getafix I think we can be a bit lax with thank you comments... there are worse things to deal with here, and thanks always makes people feel good, at least I know I like hearing it. Some people just don't upvote and don't accept, especially for newer users.. it happens.

I took the sentence from Google Books, it must be okay, I thought

> Minimal sample volume: 20 µL. (Or "minimum"?)

4:00 AM

@orthocresol I see. Sure, I'll keep that in mind

minimum i think..?

Thanks!

minimal seems to imply barely adequate..

Because in Russian it is an adjective (minimal).

One always tends to reflect one's own language

yeah, I am guessing the mean to say you need a (minimum) of 20 microlitres

Oh I see.

Yeah indeed

4:21 AM

Is it "wipers for equipment" or "wipes for equipment"? My glossary has both O_O

4:55 AM

if it is the tissue/towel like thing..then wipes

@getafix thanks!

5 hours later…

10:19 AM

@Jan I did huh, you know

4 hours later…

2:01 PM

I'm a computer programmer interested in Computer Aided Drug Design

Anyone have any advice?

Hi @ADH! What kind of advice do you need? Where to start from?

user116211

Hey Hey @Wildcat!

@MAFIA36790, hi there!

O/

user116211

Nice to see you ;))

2:17 PM

0

A: Thermodynamics Question- Entropy of Processes

I don't know why all the simpleton others are downvoting this perfectly good question- compensating for a lack of something? Anyways I would recommend breaking it into two steps- a cooling at constant pressure of the nickel, and the phase change of the ice.

the gall..

user116211

@getafix What's that?

read this guy's answer/nickname..

user116211

@getafix ohh ;P

yeah..

0

Q: Predict which hydrogen is more acidic in the following organic compounds?

I'd say the H on the second one is more acidic as the $\ce{-COOH}$ group is directly attached to the aromatic group. Am I right? Please help. It appeared on my exam today and I am intrigued. Thanks in anticipation

organic-chemistry

which one is it lol

i'm thinking maybe the left one has induction

2:24 PM

You can just look up the pKas if they are available lol

@Wildcat Yes

@getafix might not be easy to look for pKa of an ion

which is why I said if they are available lol

but what do you think about this?

yeah given that in the second the benzene ring is electron donating through the pi bonding network, and the negative charge cannot be stabilised by resonance

i'd say the first one probably, because cl is electron withdrawing

2:29 PM

@getafix cannot be stabilized by resonance? so PhCOOH is not stablizied by resonance?

@getafix I think what they want the students to see in the second one is the intramolecular H bond

user116211

@getafix There is (was) a user in Phys.SE who named himself IHateStackExchange; maybe he has changed it now.

@ADH, well, computer-aided drug design seems to be a rather broad area of scientific of research.

6

Q: Resonance of benzoic acid

My book says that benzoic acid does not show resonance as the carboxylate ion and the benzene ring are not in the same plane due to steric effects. But there aren't any large group in the ortho positions to show steric inhibition of resonance. Then, is the info given in the book wrong? If not, w...

organic-chemistry acid-base aromatic-compounds

@Hippalectryon oh yeah that did not hit me lol

@MAFIA36790 maybe..

his profile says member since today

user116211

@getafix Let it off; might be a troll....

2:33 PM

@getafix I don't understand why BzOH is not stabilized by resonance

@ADH, there are at least two unrelated topics to master: 1) computational chemistry (molecular mechanics, molecular dynamics, density functional theory, ...) and 2) statistical techniques (regression, machine learning, ...).

@getafix no need to look up pKa of an ion. that's simply pKa2 of o-phthalic acid.

true.

the first one would be harder to find imo

i would expect there to be intramolecular H-bonding effects

exactly, the first one is hard

how do you name toluene substituted compound

2:36 PM

Okay, that is helpful.

@ADH, and since you're coming from programmers' world, I suggest you concentrate on statistical techniques rather than computational chemistry since the later will be the whole new world for you.

!!img/2-chloro-2-phenylpropanoic acid

now, if you have access to proper tools... it is not so hard to find the pka ;D

user image

I have a Bachelors degree in Pre-Veterinary Medicine

2:37 PM

yay i produced the correct compound

@orthocresol Do I need to subtract the pKa from the pKa2? sorry for stupid question

@orthocresol what are these tools that you speak of?

@getafix i eat my words. reaxys doesn't have the pka

hahah

Cat needs some help in organic chemistry! :D

29

Q: Why does hyperconjugation help for ring cleavage?

The question was to rationalize it the ring cleavage happens concerted or non-concerted. The papers "The Mechanism of the Thermal Decomposition of 1-Pyrazolines and Its Relationship to Cyclopropane Isomerizations" (Crawford 1965/1966; Original papers: here and here) have shown the loss of ...

organic-chemistry homework reaction-mechanism orbitals hyperconjugation

I was trying to understand this question

The first thing which confuses me is the following phrase:

The out-of-phase linear combination of the σ⋆ orbitals looks like a p-orbital, therefore I expect a through-space interaction.

user image

2:39 PM

WTF?!

@Hippalectryon ah, you win ;)

is that a predicted pka, or measured?

@Hippalectryon what was that?

@ADH well, computational chemistry still will be the whole new world for you. :)

Can I learn what I need to know about it online? On sites like Kahn Academy?

user image

2:41 PM

@Hippalectryon how/>

@orthocresol predicted

O.o

@Hippalectryon If I have pKa and pKa2 then how do I know the pKa of the intermediate?

sorry for stupid question

@Hippalectryon I think you forgot a double bond and an O

it was supposed to be a diacid

well i disagree with the second one, the pthallic acid pKa according to wiki is 2.89, 5.51

@Wildcat I am looking at it, hold on... No guarantee I will understand it though.

2:43 PM

@user34388 I'm not sure I understand your question, I didn't follow the whole conversation before; what do you call the intermediate ?

@getafix the above (3.72) was a typo

yeah I noticed.

@Hippalectryon I mean, if I have HOOCRCOOH

I have pKa and pKa2

also @user34388 what do you mean by pKa o the intermediate?

then what is the pKa of HOOCRCOO- ?

2:44 PM

that's pKa2

@ADH I would say yes, you can. There certainly some courses (including MOOC ones) on both computational chemistry and statistical techniques. Although, I'm not sure about their quality and suitability for self-education...

I see

user image

5.51

pKa1 is first deprotnation, pKa2 is second deprotonation..and so on

2:44 PM

close match

@orthocresol Good, I'm waiting! :)

So I can write up an answer

Oh awesome! so yeah it was the first one

sighs

@getafix maybe it's better for you to write

I don't know those electron-donating terms

2:45 PM

Sure about that?

yes

@Hippalectryon teach me your magic tricks

@Hippalectryon (just out of curiosity) would you calculate the pKa of the odd hydrogen in (H3C)CH ?

@user34388 I'm not sure what you mean by "the pKa of ..."; the pKa exists for an acid-base couple, not for a single molecule

user116211

@ADH You need to know classical mechanics, electromagnetism, non-relativistic QM; linear-algebra, abstract-algebra :- Group Theory, Functional Analysis, Fourier Decomposition, Calculus, and lots of PDEs.

2:46 PM

by the "odd hydrogen" I meant the last hydrogen in the formula

And HOOCRCOO- is a member of two acid-base couples

user116211

Did I forget to add etcetera?

@getafix chemicalize.com

Are these CADD systems considered useful?

@Hippalectryon Thanks :)

2:48 PM

@user34388 (H3C)CH ? What is that ?

user116211

@ADH, Read some serious literatures rather than watching some videos.

Aren't you missing some hydrogens here ?

@Hippalectryon oops, (H3C)3CH

PEBKAC

I am in Philadelphia. I can walk down to the UPenn and buy books at the book store.

user116211

@ADH That's good!!

user116211

2:50 PM

SE has the Big List for recommendations....

@user34388 71

@Hippalectryon no pic? :(

sci-hub.cc/http://pubs.acs.org/doi/abs/10.1021/ja00435a070 @user34388, the engine isn't able to predict the pKa

@Hippalectryon Oh, I meant using your miracle software

the pka is 53. evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf

2:52 PM

@Hippalectryon because it is too high?

@orthocresol I call it experimental error lol

that should be the first recourse whenever one needs to look up an organic pka.

Not sure why :(

!!img/cyclopentadiene

user image

any aliphatic ~~hydrocarbon~~ organic compound with considerably low pKa?

2:53 PM

CpH has pka 16.

without carboxylic group

@orthocresol Yeah, I saw that

wait, did you mean !!img/cyclopentane ?

No result found.

!!img/cyclopentane

user image

3

Q: Can a hydrocarbon organic compound contain an ionic bond?

Can hydrocarbon organic compounds have a type of bonds other than covalent bonds?

organic-chemistry bond ionic-compounds hydrocarbons

aliphatic, certainly not

thats asking a bit much, isnt it

2:55 PM

haha

did generate quite a few good answers though

@Wildmeow, I think this is my interpretation:

user image

Where can I find the Big List?

@orthocresol let me think...

which looks somewhat like a p orbital on the ring carbon, albeit a bit bent.

Now I have no idea what the through space interaction is gonna be.

:D

oh shucks, i forgot to add the compound numbers when i was editing.

@orthocresol @Hippalectryon can both 53 and 71 be right?

like, the difference is so small

3:04 PM

53 and 71, small difference...?

@user34388 I'm not sure about my 71, I skimmed through the article

just 18 pka units difference..

@orthocresol like, 10^-53 and 10^-71, who can detect

0

Q: A book like Feynman's Lectures to Physics, which deals with Chemistry and provides insights.

I have been reading The Feynman Lectures, and it provides a lot of basic knowledge in physics. I would like to know if there exists a book for chemistry like this too.

Wasn't there a post about book recommendations/learning resources..

@Hippalectryon it may be obtained in a different manner

i'm not particularly inclined to look it up right now

3:05 PM

found it

@orthocresol Yeah it's a special method

@Hippalectryon Could you quote the revelant passage maybe

@Hippalectryon Can your engine compute pKa of (CH)5H?

_With our technique the only uncertainty
is the reliability of bond dissociation energies and
reduction potentials, and any solvation effects not mimicked
by the reference compound. It is thus relevant to compare our
estimated pKa of 71 for isobutane tert-butyl anion with our2
estimated pKa of 74 for trimethylcyclopropene trimethylcyclopropenyl
anion._

@user34388 (CH)5H ??

@Hippalectryon !!img/cyclopentadiene

user image

3:07 PM

Oh that

Is the linked question be on-topic though?

lol...

Nah, it seems that my engine can't compute the pKa of compounds with only C and Hs

@getafix probably not...

the books tag doesn't tell me much..

yeah thought so

3:08 PM

@Hippalectryon Is it that your engine only computes pKa of O-H bonds?

book requests are always a grey area.. here, i would just close as a duplicate of the resources thread.

How does HCCH have a pKa of 24... that's way too low for a hydrocarbon??

I can't imagine HCC- lol

i don't think there is quite anything analogous to feynman lectures

basically, just find your favourite gen chem text and read it

From a different engine : @user34388

user image

@Hippalectryon that's actually very accurate

3:09 PM

@user34388 I'm fairly sure it can do pretty much any bond except C-H, S-H and the like

@Hippalectryon try ammonium?

@orthocresol OK, now. What is hyperconjugation? According to Wikipedia, it is "the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital, antibonding σ or π orbital, or filled π orbital, to give an extended molecular orbital that increases the stability of the system."

@Hippalectryon can that engine calculate isobutane?

@user34388 works, pka 8.86

@Hippalectryon it's 9.2 ...

3:10 PM

@Wildcat mmhm, that is what it is.

@Wildcat Sounds right to me..

@orthocresol But what this out-of-phase linear combination of the σ⋆ orbitals has to do with hyperconjugation?

user image

I am not sure.

@Hippalectryon what about N=N=N-H ?

@Hippalectryon I think that's... inverted... shouldn't C2 be lower?

3:12 PM

So, that out of phase combination of C-C/C-H sigma* is the empty orbital

@user34388 back in 3 mins

@Wildcat might be useful? princeton.edu/chemistry/macmillan/group-meetings/…

Something is donating into it , perhaps the C-N sigma bond.

Sorry, can't offer much more than that link right now..

I guess that, OP is suggesting that there is a secondary orbital interaction during the reaction

3:14 PM

@orthocresol wait, before we start to look for a donor, where does this out-of-phase linear combination came from? Is it just a random guess to have an acceptor orbital of the "right" form so that some other orbital could donate to it?

As far as I can tell, yeah.

@orthocresol suggesting on the basis of what? His/her imagination? :D

I can't quite tell. Ordinarily if someone mentioned hyperconjugation to me, I would imagine that the C-H sigma orbital is the donor

@user34388

user image

@Hippalectryon 4.7, quite close

3:17 PM

That way, one can directly compare the effect of isotopic substitution, because the C-D sigma orbital is a weaker donor.

@Hippalectryon Could you use the second engine to find chloroform? This time I can't find it from the table...

there is only one important thing about chloroform... and it has the value 7.26

@orthocresol why "one important"?

> Acidity (pKa) 15.7 (20 °C)

wikipedia

user image

wow, off by a quadrillion

pentavalentcarbon

3:19 PM

@user34388 he's referring to the proton chemical shift in 1H NMR

maybe

@pentavalentcarbon what is 1H NMR? sorry for stupidity

pentavalentcarbon

proton-based nuclear magnetic resonance

never mind

By the way, I forgot to ping you as @ortho seal

@orthocresol exactly. In fact, I did some relatively simple calculations and there is a donor-accepter interaction in this molecule that can be called hyperconjugation. But not where OP expects it. Rather C-H sigma bonds on both sides of the molecule donate into N-N pi* and sigma* bonds.

pentavalentcarbon

@user34388 Don't give up so easily... en.wikipedia.org/wiki/Nuclear_magnetic_resonance_spectroscopy

3:21 PM

@pentavalentcarbon there is a second important number, that is 77.2 :D

user116211

@getafix It's Feynman's Lectures on Physics....

@ortho seal so is 7.26 right or is 15.7 right?

pentavalentcarbon

@orthocresol uhhhhhhhhhhhhh....is there a 'K' on the end by any chance?

@pentavalentcarbon haha, 13c NMR

and the pka perhaps is the 3rd most important number about chloroform

pentavalentcarbon

eh, I haven't done NMR in years, forgive me

3:22 PM

shakes head

pentavalentcarbon

go back to your corner!

organic masterrace

pentavalentcarbon

sigh

is 7.26 right or is 15.7 right?

@Wildcat That seems more logical. Are you planning to write an answer?

3:24 PM

@pentavalentcarbon Oh, 7.26 was not the pKa?

pentavalentcarbon

@user34388 no...

yep, you see how stupid i am

@Hippalectryon Why is it off by a quadrillion?

pentavalentcarbon

Blame the seal, it brought up NMR.

haha, sorry.

@user34388 Uh what are you talking about ? (I'm not following the whole conversation all the time)

3:25 PM

@Hippalectryon your computation

it is off by a quadrillion

Which one

wikipedia's value is Acidity (pKa) 15.7 (20 °C)

chloroform

your value is 30.7

exactly a quadrillion lol

@orthocresol Not yet. Oh, wait. I found some other donor-acepter pair which could possibl qualify as hyperconjugation as well. And it exactly where op expects it: at the central position. So, C-H sigma bond from methyl donates into C-H sigma* not from methyl at the same C atom.

@Wildcat I don't know whether that would affect the rate

@Hippalectryon your website is quite accurate in some...

but I still trust wikipedia more

why did you remove it?

3:28 PM

@user34388 web.missouri.edu/~glaserr/212f96/GTQ_carbenes_AK.pdf better source

(b) The pKa value of chloroform is about 25

so wikipedia is wrong :o

@Hippalectryon right?

Yeah looks that way

But I don't have a reliable source for either value

As in a reviewed article

@ortho seal You said pKa is an important number for chloroform... so do you have the value?

!!img/squaric acid

...

user image

stabilized by resonance again?

3:35 PM

Yeah

@orthocresol I have no idea why OP thinks hyperconjugation matters here.

@Hippalectryon I found a secondary source which claims 15.5 but I am unable to access the primary sources

@Hippalectryon What is HCCH stabilized by?

@user34388 Nothing special ?

@Hippalectryon but it is 24...

I meant HCC-, if you did not understand

Not sure, probably has to do with molecular orbitals, I don't really have time to look further into it

3:42 PM

alright

maybe this helps: PhC#CH is 19

so somehow the Benzene ring helped

through-space interaction
The orbital interaction that results from direct spatial overlap of two orbitals.
IUPAC Compendium of Chemical Terminology 2006

@Wildcat ???

which two orbitals?

That the whole definition. :D

are you answering my question?

No, I'm trying to understand this one chemistry.stackexchange.com/questions/33450/…

3:46 PM

Alright

This is the first time I encountered linear combination of molecular orbitals...

@Wildcat :D

No, seriously. WTF is going on out there? :D

@Wildcat I think you should start from hydrogen

@user34388 atom or molecule?

3:52 PM

@Wildcat hydrogen atoms forming molecule

@Wildcat I would tell you everything, if only I knew what was going on..

@user34388 well, they certainly do. And this can be describe by LCAO-MO formalism.

@Wildcat Wait, oh, I misred.

I thought you said this is the first time you encountered LCAO

I mean, it is one of the most-upvoted questions without an answer. I don't think anybody really knows what's going on.

But in the question, OP is combining MOs further. Not AOs, but MOs.

@orthocresol interestingly, I found that I myself also upvoted it. :D

3:54 PM

@Wildcat So did I.

And now I read it and don't understand.

@orthocresol Herd instinct? XD

@Wildcat I figured it was a good question which I didn't understand (after all there are plenty of things I don't understand here)

Obviously changing H to D has an effect, so there is basis for a question, but OP's theory as to how it comes about is ??

@orthocresol lol, that's why I often upvote OChem questions. I simply have no idea what is going on. :D

@ortho seal Do you have any idea how HCC- is stabilized?

5

Q: Relative acidities of alkanes, alkenes, and alkynes

How does one explain the trend in bond lengths and acidity of the following hydrocarbons? $$\begin{array}{ccc} \hline \text{Species} & \ce{C-H}\text{ bond length / Å} & \mathrm{p}K_\mathrm{a} \\ \hline \text{Ethane, }\ce{C2H6} & 1.102 & 50 \\ \text{Ethene, }\ce{C2H4} & 1.085 & 44 \\ \text{Ethyne...

organic-chemistry acid-base bond hybridization

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The Periodic Table

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