Organic Chemistry At a lower NMR spectrometer operating frequency, the leaning i

Question

Organic Chemistry

At a lower NMR spectrometer operating frequency, the leaning in AB systems will be stronger. In a X_2 CH_A-CH_B Y_2 fragment, the most stable conformation will generate a small 3 JAB value. Short reaction times at low temperatures favour the fastest-forming product. In enolate alkylations, one equiv. of strong base is preferred to avoid over-alkylation. In enolate acylations, a non-nucleophilic base is preferred to avoid side-reactions. Using enamines in alkylation avoids regioisomers. Intramolecular reactions are innately slower than intermolecular equivalents. Magnetic in equivalence occurs in 4-spin systems when the 4 spins occur in two pairs of two. beta-Eliminations can only proceed under basic conditions. Direct alkylations of low-pKa carbanions take place readily because metal-halogen exchanges will not occur in those cases

Explanation / Answer

1. The answer is NO. If two groups of spectral lines have similar chemical shifts then the relative line

heights may not be in the predicted ratio. The intensities of the line on the sides of

the peaks that are closest together can be greater than predicted. This is known as

‘roofing’. So roofing effect depends on chemical shifts. The chemical shifts of a given proton at low field and high field are same, because we divide by a different operating frequency in each case to determine the chemical shift.

2. True. In the stable conformation, dihedral angle between two C-H bonds are 60o. When two adjacent C-H sigma bonds are orthogonal(90o) , there should be minimal overlap, with little or no spin interactions between the electrons in these orbitals. AS a result, nuclear spin information is not transmitted, and J value approaches to 0. When the angle is 0 or 180o splitting is highest.

3. True. The product is called kinetically controlled product. If the system is heated or is left for a long time the ratio between thermodynamically controlled and kinetically controlled product increases.

4. True. In principle all the active-H can be replaced with alkyl groups. If one active H is needed to replace by one alkyl group it is desirable to use 1 equivalent of alkyl halide(commonly used electrophile) and one equivalent base to avoid over alkylation.

5. In enolate acylations , ketone enolates can be acylated with non-enolisable esters. With alkoxide bases a common problem arises is transesterification (conversion of one ester into another). This can give unwanted side products. To avoid this problem the base is normally chosen to match the alcohol portion of the ester or another alternative is to use non-nucleophilic bases because the problem arises here because the base reacts as a "nucleophile".

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