The Shapes and Structures of Molecules Pt. 1
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what is a good thing to check to ensure you are drawing tetrahedral molecules correctly
draw a flat line across the point where the two lines in the plane of the paper meet, split the molecule into quadrants, if it’s a successful tetrahedral shape, there should be one bond in each quadrant
what are two things to note generally when drawing skeletal formulae of molecules
• your structure needs to be consistent with which bonds ‘face’ which way, i.e. which direction you are viewing the molecule from • if you write a carbon, you MUST write everything attached to the carbon as well
what do we need to remember when drawing triple bonds
they are linear, unlike double bonds
what are the common abbreviations for methyl, ethyl, propyl and butyl
Me = methyl = -CH3 Et = ethyl = -CH2CH3 nPr = normal propyl = -CH2CH2CH3 iPr = isopropyl = -CH(CH3)2 nBu = normal butyl = -CH2CH2CH2CH3 tBr = tertiary butyl = -C(CH3)3
What are the common abbreviations for acetyl, phenyl, alkyl and aromatic group (aryl)
Ac = acetyl = -CH3CO(X) Ph = phenyl = -C6H5 (benzene group) R = alkyl (usually) Ar = any aromatic (aryl) group e.g. substituted benzene ring
Why do we need to be careful when using lines between molecules in inorganic chemistry
they usually don’t represent a shared pair of electrons, simply some connection between atoms
why is it the case that although O- does have a truly negative charge, the oxygen on OH- doesn’t
• we sometimes assume O in OH- is negative because we assume the electrons are shared equally between atoms through covalent bonds, this is not always the case as O is more electronegative than H
what is the name for assuming charges on certain atoms even when this might not be the true case and how do we calculate it
Formal charges, when calculating these we omit the fact that atoms may have different electronegativities to calculate formal charge:
what is the name for assuming charges on certain atoms even when this might not be the true case and how do we calculate it
Formal charges, when calculating these we omit the fact that atoms may have different electronegativities to calculate formal charge: 1) count number of electrons Ne by counting one for each bonded pair and two for each lone pair 2) calculate valence electrons associated with the neutral atom through its group Nv 3) the formal charge is (Nv - Ne)
what are some of the trivial names of common solvents
Propanone = acetone Ethanoic acid = acetic acid diethyl ether = ether methylbenzene = toluene tricholoromethane = chloroform
what occurs when X-rays are focused at a crystal and why
they are diffracted because the interatomic spacing of the atoms in the crystal is comparable to the wavelength of the x-rays
what can be deduced by analysing the diffraction pattern
• the positions of atoms within the crystal • bond lengths and angles • how molecules are arranged and pack together
what diffracts the x-rays and thus what is actually produced
the electrons in the molecules rather than the nucleons, so it actually produces an electron density map
what doesn’t show up well on x-ray diffraction and why
hydrogen atoms don’t show up on X-ray structures because they have very low electron density
what do greater contours suggest on an x-ray diffraction pattern
• greater electron density • often implies greater electronegativities or more electrons • contours join regions of = electron density
what are the main advantages and disadvantages of x-ray diffraction
Advs: - ultimate method for structural information Disadvs: - Need good quality crystals - Sometimes hard to locate atoms
what is the crudest/original method to ionise molecules
fire high energy electrons at the vapourised sample, they ‘knock’ electrons off of the molecules
what is the more gentle/effective way to ionise molecules
electrospray, - a sample is introduced as charged aerosol droplets - the solvent evaporates leaving charged molecules - what is detected is not the M+ ion but the molecule with the ion stuck to it
what is important to remember when looking at a mass spectrum
• use the specific masses of the isotopes not the average molecular mass
how can we differentiate between molecules or fragments which have very similar/the same Mr using mass spec, what else can it be used for
• take a very high resolution mass spec • use exact mass values and account for the electron loss/gain • e.g. N isn’t just 14, it’s 14.00307gmol^-1 • this can also be useful to identify large molecules where there are a large range of possibilities of structure
what is fragmentation in mass spectrometry and what are the products of fragmentation
• its where the ionised molecule becomes unstable and breaks apart • it will form a smaller ion and a radical
how can fragmentation be useful
• it can give clues to the structure of the molecule • each molecule will have a unique fragmentation pattern, this means the patterns can be used for analysis and identifying compounds whose spectra have already been recorded
what is MS/MS
• it is possible to isolate ions formed from the initial fragmentation in a mass spectrometer • these can then be ‘followed’ and it can be observed how these fragment and what they form on a mass spectrometer
Give the advantages and disadvantages of mass spectrometry
Advs: - gives molecular formula - excellent for analysis of mixtures, the components of the mixture can be identified through fragmentation patterns - tiny sample needed, mass spectrometers are very sensitive Disadv: - often difficult to interpret