exceptions in organic chemistry!!!!!

well....organic a nightmare for many fun for others....a house of many exceptions to basic phenomenon n reactions is the beauty of ORGANIC CHEMISTRY!!!!!!!
so this thread is dedicated to those exceptions in organic chemistry!!!
both in reactions n in characteristic features of compounds.........hope this thread blossoms n goes ahead[1][1][1]

36 Answers

21
govind ·

Nice idea Anand.. : )

Dipole Moments
CH3Cl > CHCl3
CH3F < CH3Cl ...

CCl3CHO can exist in it's hydrated form ie CCl3CH(OH)2
Cyclopropanone also exists in it's hydrate form due to large angle
Formaldehyde can exist in it's hydrate form..

Though F is more electronegative than Cl but still deactivating power of Cl > F

CF3 substitued on benzene will act as a meta directing grp..

see this thread one more exception given here :
http://www.targetiit.com/iit-jee-forum/posts/will-it-undergo-iodoform-reaction-15334.html

21
govind ·

NR3+ is more deactivating than NH3+...

2,4 DNP is also known as Brady's reagent

Ester Hydrolysis in the presence of acid is a pseudo first order reaction and in the presence of base is a second order reaction..

3 Hydroxy Butan-2one can show positive Tollen' s and Fehling's solution test..

C≡O is more stable than C≡C ..

1
abhishek sahoo ·

esters having one α - hydrogen do not undergo claisen condensation reaction..........

however with strong bases such as triphenylmethyl sodium, some esters like ethyl isobutyrate undergo claisen condensation ..

1
abhishek sahoo ·

not an exception but it's good to know these----

1
hacker ·

[1][1][1]nice work friends[1][1][1].thanx to u all......

P.S.yes,abhishek.......no such thing tht u only need to post exceptions any such thing which is rare to find n notice+exceptions+other properties+more........caN BE ADDED[1]

1
Ankur ·

@abhishek : for Claisen condensation : I didn't know that! But how can this be true. I mean, there is well defined reason why atleast 2 alpha hydrogen is required for Mr. Claisen... How will it attain equilibrium going with that fact..

1
Ankur ·

This one is not exception, but it's worth mentioning:
tertiary halides reacts with LiAlH4 to not-to-give alkanes; They give alkenes.
Similarly, primary halides do not react with NaBH4 at all.
To solve this trivia out, another reducing agent is used: TPH (Triphenyl tinhydride), it manages to convert every kind of alkyl halide to corresponding alkanes.

2) Boiling Point of alkanes VS Number of C atom is a linear curve, while Melting Point VS Number of C Atoms is zig-zag.. i.e, alkanes with even number of C atoms have higher mp then odd C atom chain.

1
Ankur ·

glycol reacts with dehydrating agents differently,
with (ZnCl2 + heat), glycol dehydrates to form acetaldehyde
with (H3PO4 + heat), glycol dehydrates to form diethylene glycol.

11
Pritish Chakraborty ·

LiAlH4 se yaad aaya...it can reduce all groups except double/triple bonds. HOWEVER, if a phenyl group is present beta to the double bond, it reduces the double bond as well.

96
Asish Mahapatra ·

to last post

not only 3-hydroxy butan-2-one.. but all α-hydroxy ketones give tollen's and fehling's test.

1
Ankur ·

Not only, LiAlH4, NaBH4 works similarly.. but I guess, NaBH4 reduces double bond when double bond is in conjugation.
and...
Either Sodium in liquid NH3 or LiAlH4 are considered best reagents to convert alkyne to alkene.. coz they don't reduce them further.

1
sameer puri ·

Also a good exception.

Both twist boat and half chair form of cyclohexane are chiral

1
sameer puri ·

And also schiff base give tollen test+ve.

Schiff bases= - C=N-OH

1
sameer puri ·

Also methanol is exceptionally more acidic than water. And also methoxide is better leaving group than hydroxide.

1
vipin yadav ·

please keep posting guys. its very helpfull for us. the exams are near. also please make a similar one for inorganic. thanks in advance ;p

Anonymous ·

LiAlH4 gives azobenzene (Ph-N=N-Ph) on reaction with aniline(Ph-NH2).

3
rocky ·

11
Pritish Chakraborty ·

An exception to pinacol-pinacolone rearrangement -:

11
Pritish Chakraborty ·

Weird Compounds which give/don't give iodoform -:

21
govind ·

Pyridine behaves like a deactivated benzene and does not undergo electrophilic substituions easily...there are more chences of nucleophilic substitution in case of pyridine..

whereas pyrolle behave like an activated benzene and undergo electrophilic reactions faster than benzene and the attack is generally on the carbon just next to Nitrogen atom...

Decreasing order of acidic character

Ph-SeH > Ph-SH > Ph-OH
thiophenol Phenol

p-NitroPhenol > o-NitroPhenol

o-FluoroPhenol > p-FluoroPhenol

Aniline does not undergo Friedal Crafts reactions..

Aromatic character of thiophene > furan
and pyrole > furan

1
Ankur ·

Nice Thread! Keep it going.
I'll not talk of exceptions here coz I can't remember one, but these are from my notes.
H2/Metal reduces every carbonyl, carboxylic group but H2/Pd neither reduces COOH group nor -CO- group.
In glucose, cystosine and thymine are pyrimidine bases.
phthalic acid reacts with resorcinol to give intense green orange flourescene called fluorescein dye. This is a test to detect ortha-dicarboxylic group.

1
Ankur ·

Guys! I know it's discussed but can you confirm that CCl3CHO doesn't go cannizaro reaction.

1
hacker ·

@govind,pritish and ankur......

thanx for excellent response[1][1]

keep more of it coming...here[1][1]

11
Pritish Chakraborty ·

Ankur you're right chloral undergoes HALOFORM reaction in basic medium instead...as halogen-substituted alpha carbon of aldehyde/ketone can respond to haloform reactions(if the case is like this).

Another exception to Cannizzaro is HC(triple bond)CH-CHO....it forms acetylene and formate anion instead.

1
Ankur ·

Thanks for that Pritish; I didn't know that.
Next: Keratin, a structural protein is present in Skin, Hair, and wool.
PS: It was asked in TS, with multiple answers.

1
abhishek sahoo ·

H2CrO4 DOES NOT OXIDISE DOUBLE AND TRIPLE BONDS LIKE KMnO4 OR K2CR2O7

1
abhishek sahoo ·

CH≡CH ON OZONOLYSIS HYDROLYSIS GIVES GLYOXAL AND FORMIC ACID

1
Ankur ·

glyoxilic acid gives tollens' test but not fehlings.
reaction of NH3 and formaldehyde gives urotropine.

13
Avik ·

Ok, so SUMMARISING ALL ABSURD THINGS GIVING/NOT GIVING TOLLEN's

Those which give-
*Benzaldehyde
*Formic Acid (gives Fehling as well)
*Fructose
*Alpha-Hydroxy Ketones
*Terminal Alkynes (Not a silver mirror but a white/yellow ppt.)
*Glyoxillic acid (frm ankur's post abv,,,though i dunno wht it is :P)

Those which don't-
*Tertiary Alcohols
*Compounds with any lone pair giving atom adjacent to the Carbonal Group (like amides, acid halides, esters etc..) (THis expl. is more fr Iodofrom)

(Also, see #4 abv,fr Haloform reac.n exceptions)

Tell me if there are more to add...we'll add here itself ; sab ek-saath [1]

1
Bicchuram Aveek ·

ANKUR ... WHAT IS TS ????

Your Answer

Close [X]