Because the lone pair of electrons on the Nitrogen causes the molecule to resonate, making the overall Carbon-Oxygen bond weaker, and the stretching frequency to decrease.

Two signals at 3,400cm^-1, one for each N-H bond

(less than the stretching frequency of the O-H bond, 3,500cm^-1, because N is less electronegative than O)

1,000-1,200cm^-1

(this is slightly less than the C-O bond in alcohols because N is less electronegative than O)

What is the stretching frequency for a Hydrogen bonded to a Carbon triple bonded to a Carbon?

(H-C≡C-)

What is the stretching frequency for a Hydrogen bonded to a Carbon double bonded to a Carbon?

(H-C=C-)

What is the stretching frequency for a Hydrogen bonded to a Carbon single bonded to a Carbon?

(H-C-C-)

Stretching frequencies for the isolated bonds:

C-C

C=C

C≡C

C-C :  1,200cm^-1

C=C :  1,660cm^-1

C≡C :  2,300cm^-1

Stretching frequencies for H-C bonds:

H-C-C

H-C=C

H-C≡C

*Bonus: what are the LCAO's formed for each bond?

H-C-C :  3,000cm^-1 : (sp³)

H-C=C :  3,100cm^-1 : (sp²)

H-C≡C :  3,300cm^-1 : (sp)

What are the stretching frequencies in an alcohol?

R-C-O-H

C-O : 1,100 - 1,300cm^-1

O-H : 3,500cm^-1

What are the stretching frequencies in a primary amine?

R-C-N-H
     l
     H

C-N : 1,000-1,200cm^-1

N-H : 3,400cm^-1

N-H : 3,400cm^-1

What are the stretching frequencies in an ester?

O
ll
      R-C-O-R

C=O : 1,765cm^-1

O-R : 1,100-1,300cm^-1

What are the stretching frequencies in an aldehyde?

O
ll
R-C-H

C=O : 1,710cm^-1

C-H : 2,700cm^-1 and 2,800cm^-1

What are the stretching frequencies in a carboxylic acid?

O
ll
    R-C-O-H

C=O : 1,710cm^-1

O-H : 2,500-3,300cm^-1

What are the stretching frequencies in a primary amide?

O
ll
      -C-N-H
       l
       H

C=O : 1685cm^-1

N-H : 3,300-3,500cm^-1