Week 4 Module 2

Question 1

What can Mass spectronomy indicate?

Answer 1

Molecular size, formula, and fragments.

Question 2

What can Ultraviolet spectronomy indicate?

Answer 2

The presence of conjugated systems.

Question 3

What can Infrared spectronomy indicate?

Answer 3

The presence of some functional groups.

Question 4

What can Nuclear Magnetic Resonance spectronomy indicate?

Answer 4

The number, type, and arrangement of H, C, and other atoms.

Question 5

What is an empirical formula?

Answer 5

The lowest ratio of atoms in a chemical formula. Can be determined from a combustion analysis.

Question 6

What is a molecular formula?

Answer 6

The total amount of each atom in a molecule. Can be found using the empirical formula and the weight of the molecule.

Question 7

How does Elemental Analysis work?

Answer 7

We take a sample, combust it (usually C, H compounds). When burnt, C becomes CO2, H H2O, N N2, and S SO4. We can then measure the masses and determine the percentages of C, H, N, S, in the sample. Oxygen is difficult to calculate so it is added using the missing percentage. Results in percentages of each thing present in the sample.

Question 8

What must you check for in an elemental analysis?

Answer 8

Check that the percentages add up to 100 or else add O for what is missing.

Question 9

What do you do with the percentages given from an elemental analysis?

Answer 9

Assume there is 100g of the sample and then use n = m / Mw to find the mole ratio. Divide the mass by atomic weight. (Mole ratio could be molecular formula. Does not include fractions). Normalisation is done by dividing by the smalled number. Compare the weight of the EF to the MF to get the final formula.

Question 10

How do you determine the molecular formula with mass spectronomy?

Answer 10

Mass spectronomy is used to get mass - The highest mass peak corresponds to the molecular ion M+. Works with sample being put through a beam of electrons, knowing another electron out the sample. Left with a cation radical. Because e- is so small, mass doesn’t change much. Collisions result in ionisation of the molecule (M+). Smaller mass -> deflected more. Therefore, can get a reading with different masses seperated.

Question 11

How does Mass Spectronomy work?

Answer 11

Other peaks after M+ are fragments. When you knock an electron out, it does not stay together. It is unstable and tends to break apart. Difference between molecular ion and fragment give idea of what broke off. These fragments can be used to find structure.

Question 12

What is the deflection in mass spectronomy dependent on?

Answer 12

The mass to charge ratio. In most experiments, the charge is 1, and tmass recorded is mass of molecular ion.
Can do charge of 2 depending on the indo wanted.

Question 13

How is the tallest peak referred to in mass spectronomy spectrums?

Answer 13

The base peak. Given the value of 100 and everything is measured in reference - relative abundance.

Question 14

What is the highest mass in mass spectronomy spectrums?

Answer 14

The molecular ion. There is often a smaller peak above the molecular ion to account for isotopes (with a different number of neutrons).
eg. In C7H7NO, the are isotopes of all atoms so the mass of 121 is based on the most common isotope.

Question 15

How do you find the intensity of isotopes in mass spectronomy?

Answer 15

Eg. C7H7NO. Around 1.1% of all C is C13. 0.38% of N is N15. Since a lot of sample is used, it’s almost guarenteed there are isotopes mixed in.
The intensity of M+1 peak should be around (7 x 1.1 + 1 x 0.38) = around 8%.
- Use individual mass not average
There can be molecules with equal abundant isotopes (refer to page 26)

Question 16

What can be done is the mass spectrum is aquired very accurately?

Answer 16

Molecular weight can be calculated directly from M+. Known as an accurate mass measurement.

Question 17

What is the relationship between molecular formula and empirical formula?

Answer 17

MF = (EF)n
Therefore MF weight = (EF weight)n

Question 18

What can you find by inspecting a molecular formula?

Answer 18

-The number of rings/double bonds.
- The number and type of each atom in the molecule.

Question 19

How is the number of rings and/or π-bonds also referred to?

Answer 19

The index of Hydrogen Deficiency (IHD) or Double Bond Equivalence (DBE) of the molecule

Question 20

What is the Double Bond Equivalence?

Answer 20

Conceptually the same as IHD, but a different method. Number of DBEs =
No. tetravalent atoms - no. univalent atoms + no, trivalent atoms + 1.

Question 21

Give examples of tetravalent, univalent, and trivalent atoms.

Answer 21

Tetravalent: Carbon.
Univalent: Hydrogen and halogens.
Trivalent: Nitrogen.

Question 22

What is spectroscopy?

Answer 22

Molecules can absorb energy, getting excited to a higher energy state. Thus, we can measure absorption or emission to see how much energy it takes to promote it to a higher state.
Molecule absorbs radiation of specific frequencies.

Question 23

What is the energy required to promote a molecule to a higher state related to?

Answer 23

To the wavelength which is related to the frequency of the radiation.

Question 24

How many techniques used in spectroscopic structure determination fall under absorption?

Answer 24

3/4.

Question 25

What is the electromagnetic spectrum?

Answer 25

As frequency gets lower, wavelength gets longer. Different parts of the electromagnetic spectrum can be used to measure different things.

Question 26

What can UV and IR on the electromagnetic spectrum tell us?

Answer 26

UV - electronic transition (electrons getting promoted) IR - molecular vibrations (molecules under IR light move more - depends on functional groups) Both can tell us what our molecule are made of.

Question 27

What does UV spectroscopy indicate?

Answer 27

The presence of cojugated systems (effective alternating double single bonds).

Question 28

What does infrared spectroscopy indicate?

Answer 28

The presence of functional groups.

Question 29

What does NMR spectroscopy indicate?

Answer 29

The number, type, and arrangement of H, C, and some other atoms.

Question 30

What is UV-vis spectroscopy?

Answer 30

Looking at UV absorption to see if it absorbs in that UV region. Tells us about promoting an electron from eg a bonding orbital into an antibonding orbital - can measure the energy required. The wavelength / amount of energy needed to promote electrons depends onthe type of electrons being promoted. Common with double bonds: we can promote an electron from bonding to non bonding. (refer to page 29)

Question 31

What is conjugation?

Answer 31

If there are multiple double bonds/alternating double single double bonds.

Question 32

What does the presence of conjugation do?

Answer 32

Decreases gap between homo and lumo, increasing wavelength - gets into UV region.

Question 33

Where can conjugation be seen?

Answer 33

In part of a chain or a benzene ring. Conjugated systems are usually in the 220nm region.

Question 34

How do you interpret UV data?

Answer 34

Does the molecule have an absoption in the UV region? Yes: Indicates the molecule contains conjugation No: No conjugation. Minimum DBE needed: 2. If there is this + UV absoption, yes conjugation.

Question 35

What does it mean if molecules absorb in the IR region (40-50kJ)?

Answer 35

There isn't excitement in electrons being promoted but instead: vibration.

Question 36

What are the types of molecular vibration?

Answer 36

- Stretching: bonds get longer and shorter. May or may not be symmetrical. - In-plane bending: Bonds are kinda just bending. - Out-of-plane bending: Doing things like "this" -Bending, scissoring, rocking, wagging.

Question 37

What do you do if you cannot tell between N-H and O-H?

Answer 37

Can only tell with molecular formula.

Question 38

What needs to be known in regards to molecule shape eg. for NMR?

Answer 38

What is bonded to what, does the molecule rotate, does it have symmetry, etc.

Question 39

Explain Valence Shell Electron Pair Repulsion Theory.

Answer 39

The shape of molecules can be explained and predicted by repulsion between pairs of bonding and non-bonding electrons.

Question 40

What does molecule shape determine?

Answer 40

Reactivity, what we see in the NMR spectrum and how we determine the structure.

Question 41

What is the issue of Valence Bond Theory?

Answer 41

It predicts molecular shape well, but not chemical reactivity (determined by electron distribution). Thus bonding needs to be considered.

Question 42

How is a covalent bond formed?

Answer 42

By sharing electrons between 2 nuclei.

Question 43

Refer to a diagram using valence theory to draw methane.

Answer 43

Those four bonding regions repel each other in 3D space and we get a tetrahedral shape. This idea ignores electron orbitals and how they interact. Valence bond theory only considers the overlap of the orbitals. (in H, in each of the 1s orbitals, they share electrons cause the orbitals overlap)

Question 44

What does Molecular Orbital Theory do?

Answer 44

It goes further, including bonding and anti, and why some molecules are more stable or para/diamagnetic.

Question 45

What is the difficulty with Valence bonding and Molecular Orbital (MB??) theory?

Answer 45

The shapes of the orbitals and what it looks like within a molecule.

Question 46

What are the orbital overlap model conventions?

Answer 46

- Each electron is assigned a specific orbital - No 2 electrons have identical descriptions - Electrons occupy lowest state energy levels available (in ground state) -Only valence electrons used to describe bonding

Question 47

What are P-orbitals to each other?

Answer 47

Perpendicular.

Question 48

Explain atomic orbitals mixing.

Answer 48

When they mix (eg H s-orbital and p-orbital overlapping), what happens ot the tail end of that orbital? Potentially different energies - electrons shared into orbitals that are diffferent, so why is the molecule symmetrical? S-orbitals are spherical, p-orbitals are dumbbell shaped <- 3 perpendicular along x,y,z.