[leena]

1) 50cm³ of 1M CH₃CHO is placed in a vessel at the anode of an electrolytic cell.
Find the time taken for a current of 0.1A to flow through the external circuit.
(ans:32166.667minutes or 193000s)

So I wrote the half equation,
H₂O + CH₃CHO  CH₃COOH + 2H⁺ + 2e

Now using Faraday's 1st law of electrolysis,
Q=it
i=0.1A so to find time,t ,I need to find the charge Q

Then using Aavagadro's constant,
1mol of electrons require  96500C
therefore according to the equation,
for 2mols of electrons      2*96500C

and according to the data given 0.05 moles of CH₃CHO would give 0.1 mols of electrons,so,
0.05 mols of CH₃CHO( i.e. 0.1 mols of electrons would require) :rarrow:96500*0.1C=9650C

Substituting this to Q=it,I get t= 96500s?
What am I doing wrong?
I've checked my notes many many times,but I still can't figure it out 

2) Another question,can someone please explain to me what an allyl,aryl and a vinyl carbocation is?

Thank you


 

[plankk]

In the picture you can see carbocations in order: allyl, vinyl, aryl.

[leena]

In the picture you can see carbocations in order: allyl, vinyl, aryl.

Thank you so much,plankk.
I have another question,
is it possible to have an allyl-vinyl carbocation?If so,would this be more reactive towards water than an allyl would?


1) As for the 1st part of my question on electrolysis,I would really appreciate it if someone could help me solve it.
Thank you.

[lancenti]

I don't think it's possible to form an Allyl-Vinyl Carbocation since it'd have to look something like...
     /
=C+
     \

Which doesn't make sense since Carbon would have an octet yet a positive charge.

[leena]

Thanx
one more question,
if we had,
1.allyl halide
2.primary aryl halide
3.RMgX
4.acidanhydride
5.aliphatic acid chloride
6.tertiary alkyl halide

how would we arrange these compounds inorder of increasing reactivity with water?

I think it should be,
 tertiary alkyl halide< allyl halide < aliphatic acid chloride < primary aryl halide < Grignard's
but I don't really know how an acid anhydride would react?

[lancenti]

Amt. of Ethanal = 0.050 mol
Amt. of Electrons = 0.100 mol

Charge per Electron = 1.6 x 10^-19 Coulombs
Charge per Mol of Electrons = 9.632 x 10⁴ Coulombs

Total Charge Required for Full Reaction = 9632 Coulombs
Time Required = 96320 Seconds

I am of the mind that it's a typographical error since your working is correct. Above is what I did to get the same answer.

[leena]

Thanks for confirming.

[lancenti]

Is it right for me to assume that by Acid Chloride you mean Acyl Chloride, Ie R-COCl?

If so, it's very reactive with water as it has a very positive carbon which is susceptible to nucleophilic attack by water, hence giving it its acidic properties.

Also, Acid Anhydrides are very much similar to Acyl Halides. May want take that into consideration as well.

[lancenti]

I think the best way to look at this question, since reaction with water is usually through an electrophilic attack (water being an electrophile), you'll want to order the compounds in terms of nucleophilic strength.

[leena]

Is it right for me to assume that by Acid Chloride you mean Acyl Chloride, Ie R-COCl

yes,that's what I meant.

I think the best way to look at this question, since reaction with water is usually through an electrophilic attack (water being an electrophile), you'll want to order the compounds in terms of nucleophilic strength.

Isn't water a nucleophile,(as in nucleus loving ryt?)
Since it has extra electrons and wouldn't the OH^- act like a nucleophile and attack maybe the electrophilic C?
I'm a little confused.
Thanks

[leena]

If so, it's very reactive with water as it has a very positive carbon which is susceptible to nucleophilic attack by water, hence giving it its acidic properties.

I think you meant water is a nucleophile after all.Hope I'm right?

Arranging the compounds in order of nucleophilic strength,
tertiary alkyl halide> allyl halide> now I don't know if acid anhydride is more nucleophilic than RCOCl,
I'd like to think it is,cause an acid anhydride has 2 carbonyl groups whereas an acid chloride has only 1,but I'm not very sure?

[leena]

Could someone tell me how to do this,

if we had,
1.allyl halide
2.primary aryl halide
3.RMgX
4.acidanhydride
5.aliphatic acid chloride
6.tertiary alkyl halide

how would we arrange these compounds inorder of increasing reactivity with water?

I've tried everything I could think of,but I still can't figure it out.
I found an old thread that explained the reaction of acid chloride and acid anhydride with water,but I just got more confused,after reading it,
http://www.chemicalforums.com/index.php?topic=32754.0
could someone explain what I need to consider to find the order of reactivity with water?
Thanks in advance