IR of alkynes

Spectrum of 1-octyne:

=C-H stretch: Around 3300 cm^-1

C-C triple-bonded stretch: Around 2150 cm^-1

The triple-bonded stretch acts like the double-bonded stretch. It is lowered by conjugation and does not have a peak if the bond is symmetric

The triple-bonded C-C-H bend is in the spectra but not mentioned in the textbook. I assume it acts the same as a double-bonded C-C-H bend.

Below is the spectrum of 4-octyne. You should be able to tell why three peaks are missing compared to 1-octyne:

IR of alkenes

The spectra of 1-hexene:

=C-H stretch: 3010 - 3095 cm^-1

=C-H out of plane bending (oop): 1000 - 650 cm^-1

C=C stretch: 1660 - 1600 cm^-1

Conjugation lowers the frequency of the C=C stretch.

Cis double bonds with the same substituents do not have a C=C peak, because the stretch would have no change in dipole moment. But but the substituents are different then cis peaks have stronger absorptions than trans.

Trans double bonds with the same substituents have an extremely weak C=C peak.

The spectra of cyclohexane:

Notice how there is still a small C=C peak, meaning the C=C is not symmetric. You can see why by looking at a model:

The two images below show the effect of cis-trans isomerisation on the C=C absorption:

Notes about IR spectroscopy

• Only bonds which have a change in dipole moment during the vibration can be excited by infrared. So symmetric bonds such as H-H and Cl-Cl do not absorb infrared, while H-Cl does.
• Different, independent vibrations are called normal modes.
• A linear molecule with N atoms has 3N - 5 normal modes
• A nonlinear molecule with N atoms has 3N - 6 normals modes

For example, carbon dioxide is linear with three atoms, so there are four normal modes:

• Each vibrational mode does not have only one ground state and one excited state. There is a range of states, v₁, v₂, v₃, etc. The fundamental transition for a normal mode (v₁ -> v₂) has a much higher absorbance, these are the most common IR peaks, which A-level and early undergrad courses focus on completely. But transitions from ground state to a level higher than v₂ exist - they are called overtones:

The energy level corresponding to v = ∞ is the bond disassociation energy - it can visualised as the level where vibrations are so energetic that the bonds fly apart. The gaps between energy levels converge to 0 as they approach the bond disassociation energy.

Energy gaps lower down are approximately equal to the fundamental transition. Since wavenumber is proportional to energy, we can expect the v₁ -> v₃ overtone and the v₁ -> v₄ overtone to be respectively twice and triple the wavenumber of the fundamental transition.

When interpreting spectra, overtones can overlap with fundamental absorptions, and people unfamiliar with overtones might attribute the peak to a functional group which doesn't exist. But usually the overtones can recognised by their low intensity.

• An absorption can exist which is the sum of two others, this is called a combination band.
• A absorption can exist which is has the value of one subtracted from another, this is called a difference band.

Not all peaks can combine or subtract to produce combination and difference bands - it depends on complicated rules which are beyond the scope of my textbook.

• Stronger bonds and lighter atoms vibrate faster. This explains why the C-H stretch is typically the peak at the highest wave number. It also explains why hydrogen attached to an sp² carbon vibrates at an even high wavenumber than a hydrogen attached to a sp³ - carbon - the sigma bond of a sp² carbon is stronger, since it contains more s character.

Can you tell why the the conjugated carbonyl below absorbs at a lower frequency than a non-conjugated one?

Answer: Resonance gives the carbonyl more single-bond character, weakening the bond, hence weakening the frequency of its vibration.

Significant figures 2

A logarithm is composed of an integer and decimals. The integer is called the characteristic and the decimal part is called the mantissa.

The number of significant digits in the original number should equal the number of digits in the mantissa:

This is true in the other direction, the number of digits in the mantissa of a log should equal the significant figures of the antilog:

Lab equipment 2

Fume hood:

The inside of a fume hood has air constantly sucked out through the top, hence minimising your exposure to toxic fumes or dust. A fume hood is also used to handle non-toxic but flammable substances. If there is a fire, you can step back and pull the cover down. The oxygen and any toxic gases produced will be sucked out inside a safe environment.

If you are handling flammable volatile substances which have a vapour heavier than air, then using a fume hood can avoid the following situation:

Having gas constantly sucked upwards will prevent this fire hazard.

Fumehoods can cost over $10,000, so in undergraduate labs they may be cramped. Many students will therefore do things outside the fume hood which should technically be done inside - I suggest using your own judgement. Universities are so careful to avoid being sued that they will tend to overstate the number of chemicals which need to handled be inside the fume hood.

It is suggested to keep the cover down when the fume hood is not in use. I ignored this for about 2 years until i learnt why - fumehoods consume as much power as a small house, and this consumption is minimised when the cover is down. People don't like arbitrary rules with no explanations, but they do like to save energy.

Lab coat:

Lab coats do not seem obviously important. But over the first year I came across many small holes (such as produced by acid) and random chemical stains. It is useful to be protected from these, especially since many safe and common powders and liquids can still be toxic over a long time period. It is nice to have these absorb onto your coat and stay in a locker, rather then contaminating your ordinary clothes and having them brought home with you.

Goggles:

Goggles are probably the most stressed safety feature in any lab. Acid on your skin can give you blisters and burns, while an acid splash in your eye can easily blind you permanently.

Significant figures

Many people, even many scientists, find statistics and error analysis to be boring. But it is a necessary evil, both to get high grades and to know whether we can confidently say something is true.

From A-level we should remember that a result of 25 has two significiant figures, and a result of 25.0 has three significant figures, 0.000025 has two significiant figures, while 0.0000205 has three.

It is useful to write results in exponential form. You are likely to make mistakes when writing out multiple zeros. But more importantly it prevents ambiguity in significiant figures. Consider the number 92500, it could represent:

9.25 x 10⁴ (three s.f.)

9.250 x 10⁴ (four s.f.)

9.2500 x 10⁴ (five s.f.)

If we multiply or divide numbers, the answer is limited by whichever number has the least significant figures:

If we add or subtract numbers, it is OK to get answer with a different amount of significant figures. 

But the sum is limited by whichever number has the least decimal places:

The answer above should be rounded to the nearest significant digit, which in this case is 121.795.

To add numbers with different exponents, first convert them to the same exponential form. Then use the same rules as before.

In the above example, the answer is rounded to 11.51 x 10⁵ because the number 9.84 x 10⁵ limits the answer to two decimal places.

To remember the difference, think "multiplication = significant figures" and "addition = decimal places".