Textbook: Chapter 11.3
Links: Understanding Chemistry Virtual Textbook
Mission 1: Peep This...
Mission Objectives. You should be able to...
1. Determine the IHD from a molecular formula.
2. Deduction of information about the structural features of a compound from percent composition data, MS, IR, or 1H NMR.
Degree of Unsaturation (Index of hydrogen deficiency [IHD]) can be used to determine from a molecular formula the number of rings or multiple (pi) bonds in a molecule.
Double bonds are counted as ONE degree of saturation
Triple bonds are counted as TWO degrees of saturation
A ring is counted as ONE degree of saturation
An aromatic ring is counted as FOUR degrees of saturation
IHD can be worked out two ways: from the structure itself or from the molecular formula. Watch the below video from Richard Thornley. Be sure to write down the formula he uses, as it is easier than the one in your text.
Spectroscopy is the study of light and matter. It's used to gather data about what the structure of a molecule might be. There are two main types of light spectroscopy: IR and NMR.
Infrared Spectroscopy. IR light is passed through a sample and to a detector (see first image below). Some of the light is absorbed by the sample, some isn't, and some of the light passes through to reach the detector. IR light is used because it makes molecules vibrate in certain ways: symmetrical, asymmetrical, stretching and bending. This happens at particular energies. Very specific wavelengths of light are absorbed and it varies depending on the functional groups in the chemical sample.
Image courtesy of www2.chemistry.msu.edu
Image below is from wikipedia.
(1) A chemical shift describes the chemical environment. Anywhere there is a hydrogen atom, there is chemical information about that hydrogen's environment. The closer that proton is to an electronegative element, said element will have a de-shielding effect (electrons move away from the nucleus), the further downfield the signal will be. Examining the graph above, downfield is to the left and upfield is to the right. Downfield is closer to electronegative elements and upfield is farther away from electronegative elements.
(2) Integration is the area under the curve (what we're seeing is a grossly simplified example). How big is the resonance? How many chemically equivalent 1H (protons) are generating the peak?
(3) Splitting. Resonance will split into smaller peaks, depending on neighboring protons. This uses the n + 1 rule where "n" is the number of neighbor protons.
Professor Dave goes into detail and provides an example for you to work through and understand.