NMR 3

Question 1

EWGs will deshield and therefore shift the neighbouring H down field to higher chemical shift value. TRUE OR FALSE?

Answer 1

TRUE

Question 2

How are the hydrogens directly attached to C=C deshielded?

Answer 2

They are deshielded in two ways:

1. Pi bonds generate magnetic filed Bi that adds to Bo in the region of the hydrogens therefore deshielding
2. Spe C has high S character, therefore attracting electrons, this electron withdrawing effect is also deshielding

Question 3

What are olefinic H’s?

Answer 3

Hydrogens directly attached to C=C

Question 4

What region in NMR spectroscopy is diagnostic for aromatic signals?

Answer 4

6.5 - 8.0 ppm

Question 5

What occurs to cause the deshielding effect in aromatic signals?

Answer 5

-Circulation of electrons in the ring creates a magnetic filed Bi, so we get Bo+Bi (deshielded) in the region of the aromatic H’s

Question 6

Why are the hydrogens in an aromatic ring have high chemical shifts?

Answer 6

The induced magnetic field is not opposing so, its adding to it, therefore deshielding it.
-More S character so electrons are pulled away from H’s (deshielding)

Question 7

Why do alkynes appear at higher chemical shifts than alkenes (upfield)?

Answer 7

Because they are more shielded than alkenes

-The induced field is shielding the H’s across the triple bond

Question 8

why are H’s attached to triple bond have higher than expected chemical shift values?

Answer 8

Because they have more S character

Question 9

H’s attached to C bearing OH or primary or secondary amines, absorb in a similar region to H’s adjacent to ethers and tertiary amines. TRUE OR FALSE?

Answer 9

TRUE

Question 10

Why does O-H and N-H vary greatly between samples?

Answer 10

• Due to the extensive hydrogen bonding at these positions

- The amount of H bonding depends on the concentration of the sample

Question 11

Where can N-H and O-H be seen?

Answer 11

10-15ppm

Question 12

Chemical shift does not depend on the chemical environment (this is altered by the degree of H bonding). TRUE OR FALSE?

Answer 12

FALSE

Question 13

Less hydrogen bonding = higher field (up field shift) . TRUE OR FALSE?

Answer 13

TRUE

Question 14

H’s attached to O or N can be off scale, they tend to move around. TRUE OR FALSE?

Answer 14

TRUE

Question 15

Atoms bonded directly to electronegative atoms will have higher chemical shift value. TRUE or FALSE?

Answer 15

TRUE

Question 16

What are singlets, doublets, triplets and quartets?

Answer 16

• Singlet - made up of single line 1
• Doublet - made up of 2 lines 1:1
• Triplets - made up of 3 lines 1:2:1
• Quartet - made up of 4 lines 1:3:3:1

Question 17

At what ppm do we see Chloroform?

Answer 17

7.26ppm

Question 18

What is CDCl3?

Answer 18

Solvent used to dissolve compounds, does not produce any signals it is pure

Question 19

What is integration?

Answer 19

area under each signal