Assessment 4 Flashcards
Nuclear magnetic resonance spectroscopy
Involves interaction of nuclei of atoms with the radio waves portion of the electromagnetic spectrum
How does NMR work?
Takes advantage of the spin property of nuclei. When exposed to an external magnetic field, the spin of the nuclei will either align with the field (up spin) or against the field (down spin). The down spin is highest in energy. If a sample is radiated with a frequency that matches the change in energy states, the nuclei is excited to the higher energy spin state, and a signal is produced when the energy transfer is detected.
The difference in energy between spin states is dependent on
External magnetic field strength
Why would the same kind of proton have different resonance frequencies?
The resonance frequency depends on the electronic environment of the proton. Electron density on a proton depends on what else is attached to the atom bonded to it.
Shielding effect
Electrons surrounding the proton/nuclei in a bond generate a secondary field that opposes the magnetic field, causing a shielding effect on the nucleus. Electron donating atoms/groups increases shielding, and electron-withdrawing groups/atoms
cause a de-shielding effect.
How does shielding effect the resonant frequency?
A shielded proton/nuclei resonate at a lower frequency (upfield) and de-shielded
proton/nuclei at a higher frequency (downfield). Less electron density around a proton results in a high resonance frequency.
Chemical shift
The resonance frequency of a nucleus/proton measured against a reference (usually TMS). It is the distance in Hz or ppm from the center of a signal to the center of the reference signal
What determines the chemical shifts of protons?
The chemical environment around the proton
Signal area (integration)
The sum of the areas under all peaks in a signal and is proportional to the number of protons producing the signal. The signal area is proportional to the vertical rise of the integrator as it crosses the signal
Coupling interactions
A signal generated by a given proton or set of protons may be split into several peaks due to coupling interactions with nearby protons. Spin-spin coupling is typically observed between nuclei that are one, two, and three bonds away.
Multiplicity
The number of separate peaks in a
signal- tells us the number of neighboring protons to the one giving the sign. Multiplicity is generally determined using ‘n+1’ rule - for ‘n’ identical or near-identical neighboring hydrogens, ‘n+1’ splits are observed
Coupling constant
The distance between two adjacent peaks/splits in a signal is referred to as its coupling constant and is measured in hertz (Hz). Coupling constant is a measure of the interaction between a pair of protons. Protons that are coupled to each other have the same coupling constant
When does coupling/peak splitting occur?
Coupling or peak-splitting happens between non-equivalent protons. Chemically equivalent protons do not exhibit spin-spin coupling
What type of protons are considered chemically equivalent?
Peak splitting is not observed for homotopic and enantiotopic protons
What do the number of signals in an NMR spectrum represent?
The number of non-equivalent protons or sets of protons in a molecule. Chemically equivalent protons resonate at the same frequency.
Carbon NMR
13 C carbon isotope [accounts for only 1.1% of naturally occurring carbon] has spin property and can produce an NMR signal. Carbon NMR experiment involves high-field pulse technique coupled with broad-band proton or heteronuclear decoupling.
What do the number of signals represent in carbon 13 NMR?
Represents each unique (chemically inequivalent) carbon in a molecule
The external magnetic field experienced by a carbon nuclei [chemical shift] is affected by
The electronegativity of the attached atoms and the hybridization of carbon
2,4-Dinitrophenylhydrazine test
2,4-Dinitrophenylhydrazine reacts with ethanol and the unknown. Tests for aldehydes and ketones. Positive result- yellow-red precipitate
Chromic acid test
Unknown reacts with acetone and the chromic acid reagent. which contains sulfuric acid. Tests for primary alcohols, secondary alcohols. and aldehydes. Positive test- green precipitate
Sodium bicarbonate test
Unknown reacts with sodium bicarbonate. Tests for carboxylic acid. Positive result- carbon dioxide gas bubbles
Acetyl chloride test
Unknown reacts with acetyl chloride. Tests for alcohols, primary amines, and secondary amines. Positive result- evolution of heat and hydrogen chloride gas
Bromine test
Unknown reacts with methylene chloride and bromine. Tests for unsaturation (double or triple bonds). Positive test- dissipation of reddish brown color without the evolution of hydrogen bromide gas
Potassium Permanganate test
Potassium Permanganate reacts with unknown and water or ethanol. Tests for double or triple bonds- some aldehydes, alcohols, phenols, and aromatic amines. Positive result- loss of purple color and formation of brown precipitate
Beilstein test
Uses copper wire and an ethanol solution of the sample. The presence of a green flame in a Bunsen burner is positive for a halogen
Silver nitrate test
Unknown reacts with ethanol and silver nitrate. A white precipitate indicates a positive test. Carboxylic acids will produce a precipitate that is soluble in nitric acid. A precipitate that is insoluble in nitric
acid confirms the presence of a halogen. Benzyl, allyl, and tertiary halides give a positive test immediately. Secondary and primary halides give a positive test after heat is applied to the solution. Aryl and vinyl halides do not react at all.
What occurs during a Fischer esterification reaction?
A carboxylic acid reacts with an alkyl alcohol in the presence of an acid catalyst. The carboxylic acid is protonated by the acid, then the carbon of the carboxylic acid is attacked by the alcohol in a nucleophilic fashion. This creates an unstable intermediate that undergoes dehydration to form the ester
What is a disadvantage of a Fischer esterification?
Fischer esterification is also reversible, so the ester and water products could react to form the original carboxylic acid and alcohol through hydrolysis. According to Le Chatelier’s principle, this outcome can be prevented by using an excess of the starting reagent to shift equilibrium to the right side, which is the side of ester formation. A drying agent can also be used to remove water
What 3 techniques are used in Fischer esterification?
Reflux, extraction, and distillation
Which compounds were used in Fischer esterification?
Ethanoic acid was the carboxylic acid. The alcohol was unknown and identified by the experiment. Sulfuric acid was the acid catalyst.
