Pre-lab questions 1. Several of the possible unknown aldehydes have similar phys

Question

Pre-lab questions 1. Several of the possible unknown aldehydes have similar physical properties (appearance, melting point, boiling point, etc). How could IR and NMR be useful for identification of the unknown reagent? The self-condensation of acetone is a possible side reaction for the aldol condensation Reaction 1). Draw the structure of this side product. How is this side reaction minimized in our procedure? 3. Most imine formation reactions are performed in the presence of molecular sieves or magnesium sulfate. What is the purpose of these added reagents?

Explanation / Answer

IR- SPECTROSCOPY

ALDEHYDE GROUP HAS COMMON FREQUENCY NEAR~ 1700 Cm-1 HOWEVER IF SUBSTITUENT VARIES IT FREQUENCY CAN ALSO VARY :

(1) if aldehyde group has c=c next to c=o IT DECREASE ALDEHYDE FREQUENCY BECAUSE THERE IS CONJUGATION BETWEEN C=C AND C=O WHICH DECREASE C=O DOUBLE BOND CHARACTER C-C-C-0 FORCE CONSTANT DECREASE TO ~ 1700 Cm-1

(2) +I (INDUCTIVE EFFECT) ALKYL GROUP WILL DECREASE ALDEHYDE FREQUENCY TO ~ 1700 Cm-1

(3)

-I (INDUCTIVE EFFECT) ELECTRON WITHDRAWING GROUP WILL INCREASE ALDEHYDE FREQUENCY TO ~ 1700 Cm-1

(4)

HYDROGEN BONDING WILL ALSO DECREASE ALDEHYDE GROUP FREQUENCY ~ 1700 Cm-1 AS IT INCREASE BOND LENGTH AND DECREASE C=O DOUBLE BOND CHARACTER.

NOTE:

INTER MOLECULAR HYDROGEN BONDING WILL GIVE RISE TO BROAD BAND WHILE INTRA MOLECULAR HYDRONG BONDING GIVE SHARP BAND.

NMR SPECTROSCOPY IDENTIFICATION

IN NMR SPECTROSCOPY SUBSTITUENT SHOW UPFIELD AND DOWNFIELD EFFECT WITH RESPECT TO ALDEHYDE NORMAL

IN ALDEHYDE H-C=0 CHEMICAL SHIFT = 9.0~ 10 PPM

H- CH2- C=O  CHEMICAL SHIFT = 2.0~ 2.5 PPM

ELECTRON RELEASING GROUP SHOW UPFIELD SHIFT AND ELECTRON WITHDRAWING GROUP SHOW DOWNFIELD SHIFT.

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