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CHEM2010 - Synthesis and Spectroscopy > Topic 1 - Organic Spectroscopy > Flashcards

Topic 1 - Organic Spectroscopy Flashcards

1
Q

What is spectroscopy?

A

The interaction between matter and electromagnetic radiation.

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2
Q

What does IR spectroscopy measure?

A

The frequencies at which covalent bonds vibrate (stretching or bending).

It is used to identify functional groups.

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3
Q

What does UV/Vis spectroscopy measure?

A

The frequencies at which electrons transition between orbitals.

It is used to reveal the presence of a conjugated π system which is shown by a significant absorption above 200 nm.

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4
Q

What does NMR spectroscopy measure?

A

The frequencies at which the orientation of a nuclear magnetic dipole changes in an external magnetic field.

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5
Q

What are the three basic features in a 1H NMR spectrum?

A

Chemical shift which represents the proton environment.

Signal intensity which represents the relative number of protons in a given signal.

Spin-spin coupling which represents the number of nearby protons.

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6
Q

What does mass spectrometry measure?

A

The mass-to-charge (m/z) ratio of gas-phase ions.

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7
Q

What does a small peak at around 275 nm correlate to in UV/Vis spectroscopy?

A

A carbonyl bond.

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8
Q

A higher IR absorption frequency is caused by…?

A

A larger value of the force constant, k (a stronger bond).

A small value of the reduced mass, μ (atoms with lower atomic mass).

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9
Q

What are the four regions of an IR spectrum?

A

4000-2500 cm-1 - X-H bonds.

2500-1900 cm-1 - triple bonds.

1900-1500 cm-1 - double bonds.

1500-400 cm-1 - fingerprint region.

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10
Q

What are the features of an O-H (alcohol) stretch?

A

Very broad peak at 3200-3600 cm-1.

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11
Q

What are the features of an O-H (carboxylic acid) stretch?

A

Very broad peak at 2500-3000 cm-1. C-H peaks are superimposed.

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12
Q

What are the features of an N-H (amines and amides) stretch?

A

A broad peak between 3300-3500 cm-1.

Primary amines/amides will show two bands.

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13
Q

What are the features of a C-H stretch?

A

They are found at around 3000 cm-1.

sp3</sup stretches are found lower than 3000 cm-1.

sp2 and sp stretches are found higher than 3000 cm-1

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14
Q

What are the features of an aldehydic C-H stretch?

A

Often two bands are present at 2700-2900 cm-1.

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15
Q

What are the features of an S-H stretch?

A

A weak band at 2550-2600 cm-1. It is less broad than OH bonds.

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16
Q

What are the features of a C≡C and C≡N bond?

A

They are medium intensity peaks at 2100-2260 cm-1.

An alkyne will also possess a sp C-H bond above 3000 cm-1.

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17
Q

What are the features of a C=C stretch?

A

Weak-medium absorption at 1620-1680 cm-1.

Conjugation lowers the absorption by 40 cm-1.

sp2 hybridised C-H will also be present.

18
Q

What are the generic features of a C=O stretch?

A

A strong absorption at 1650-1800 cm-1.

19
Q

What are the features of a C=O (acid chloride) stretch?

A

A strong absorption at 1800 cm-1.

20
Q

What are the features of a C=O (anhydride) stretch?

A

2 strong bands at 1750 and 1820 cm-1.

21
Q

What are the features of a C=O (ester) stretch?

A

A strong absorption at 1735-1750 cm-1.

22
Q

What are the features of a C=O (aldehyde) stretch?

A

A strong absorption at 1720-1740 cm-1.

23
Q

What are the features of a C=O (ketone) stretch?

A

A strong absorption at 1705-1720 cm-1.

24
Q

What are the features of a C=O (carboxylic acid) stretch?

A

A strong absorption at 1700-1725 cm-1.

25
Q

What are the features of a C=O (amide) stretch?

A

A strong absorption at 1670-1700 cm-1.

26
Q

Why do carbonyls in different functional groups absorb at a different wavenumber?

A

The functional group affects the strength of the C=O bond. Acid chlorides have the strongest C=O bond whereas amides have the weakest C=O bond.

27
Q

What are the effects of conjugation with C=C or Ph in carbonyls?

A

All of their absorptions are decreased by 20 cm-1 apart from amides where it is increased by 15 cm-1.

28
Q

In cyclic ketones, what are the effects of ring size? Why does this happen?

A

A smaller ring size results in a larger wavenumber.

This is because there is increased p character in the bonds in smaller rings. This causes the C=O bond to show increased s character to compensate. This results in a stronger bond and a higher frequency absorption.

29
Q

What are the features of a C-O stretch?

A

Medium-strong absorptions at 1000-1300 cm-1.

Multiple absorptions are common.

30
Q

What are the features of an N-O (nitro) stretch?

A

Medium strong absorption at 1300-1400 cm-1.

Strong absorption at 1500-1600 cm-1.

31
Q

What are the features of a C-Cl stretch?

A

Weak absorption at 600-800 cm-1.

32
Q

What absorption is characteristic of a conjugated π system?

A

A significant UV absorption above 200 nm.

33
Q

What does a higher electron density mean for NMR spectroscopy?

A

A lower frequency will be absorbed.

34
Q

What are the ranges for functional groups in 13C NMR?

A

0-80 ppm - Aliphatic carbon
80-120 ppm - Alkyne carbon
100-140 ppm - Alkene carbon
110-130 ppm - Nitrile carbon
110-160 ppm - Aromatic carbon
160-220 ppm - Carbonyl carbon

35
Q

What are the advantages and disadvantages of using a proton decoupled spectrum for 13C NMR?

A

Advantages: Overlap between signals is reduced and there is a much better signal to noise ratio.

Disadvantage: Information is lost on how many protons are attached to the carbon.

36
Q

What does DEPT stand for?

A

Distortionless enhancement by polarisation transfer.

37
Q

What happens in DEPT 45?

A

All of the signals remain except fully substituted carbon.

38
Q

What happens in DEPT 90?

A

Only CH remains.

39
Q

What happens in DEPT 135?

A

CH and CH3 remain, whilst CH2 becomes negative.

40
Q

What are the ranges for 1H NMR?

A

0-2 ppm - Aliphatic hydrogen.
2-4.5 ppm - Aliphatic hydrogen near strong EWG.
5-6 ppm - Olefinic hydrogen.
6.5-8.5 ppm - Aromatic hydrogen.
9-11 ppm - Aldehydic hydrogen.
9-12 ppm - carboxylic acid hydrogen.

41
Q

How can EWGs and EDGs on an aromatic ring affect the chemical shift for 1H NMR?

A

EDGs will lower the chemical shift for the ortho and para positions but affect the meta position very little.

EWGs will increase the chemical shift for the ortho and para positions but affect the meta position very little.

This can be rationalised by drawing resonance forms.

42
Q

Why do alkenes have high chemical shifts even in the absence of EWGs?

A

The electrons within the π system circulate which causes a secondary magnetic field. This magnetic field is aligned in a particular way so that alkene hydrogens have an increased chemical shift. In alkynes, the effect is in the other direction.

These are referred to as anisotropic effects.

CHEM2010 - Synthesis and Spectroscopy (4 decks)

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