Molecular Structure and Shape Determination Flashcards
Calculate the empirical formula of a molecule given data from elemental (combustion) analysis.
Refer to cheat sheet. Empirical formula = original form (without multipants).
Explain the relationship between the empirical and molecular formula.
Empirical formula is simply a ratio of different atoms in a compound to each other. The molecular formula shows the actual number of atoms of each of the elements in a compound.
Calculate the molecular formula from empirical formula (determined by experimental analysis) given a molecular weight (from mass spectroscopy).
Refer to cheat sheet.
Calculate the number of double bond equivalences for a molecule given either the molecular formula or the chemical structure.
Refer to cheat sheet.
Describe the property of conjugation in a molecule.
Conjugation = system that contains alternating double and single bonds. Requires AT LEAST 2 double bond equivalences.
Predict whether a molecule will show absorbance in the UV-vis spectroscopy given a chemical structure.
If conjugation is present, molecule will show absorbance.
Predict the likely features in an IR spectrum for a molecule given a chemical structure (and a table of characteristic IR absorptions).
Refer to infrared spectra table of characteristics on provided exam sheets.
Describe the hybridisation state of atoms (sp, sp2 and sp3) based on hybrid orbital theory.
VSEPR Theory = molecule will take a shape where electronic repulsion in the valence shell of an atom is minimised.
In sp hybridization, one s orbital and one p orbital hybridize to form two sp orbitals, each consisting of 50% s character and 50% p character. This type of hybridization is required whenever an atom is surrounded by two groups of electrons.
In sp² hybridization, one s orbital and two p orbitals hybridize to form three sp² orbitals, each consisting of 33% s character and 67% p character. This type of hybridization is required whenever an atom is surrounded by three groups of electrons.
In sp³ hybridization, one s orbital and three p orbitals hybridize to form four sp³ orbitals, each consisting of 25% s character and 75% p character. This type of hybridization is required whenever an atom is surrounded by four groups of electrons.
Predict the integration of signals in a 1H NMR spectrum given a chemical structure.
Integration = the number of hydrogen atoms in a particular environment.
Predict the multiplicity of signals in a 1H NMR spectrum given a chemical structure.
Multiplicity = indicates the number of hydrogen atoms on the adjacent carbon atom(s).
Signal multiplicity = number of adjacent spins (no. of adjacent hydrogen atoms) + 1.
E.g. CH3 - CH2 - O - CH2 - CH3
CH2 signal = 3 adjacent spins, multiplicity = 4, ‘quartet’
CH3 signal = 2 adjacent spins, multiplicity = 3, ‘triplet’
Predict the chemical shift of signals in a 1H NMR spectrum given a chemical structure (and a table of characteristic NMR chemical shifts).
Chemical shift = the position of the signal indicates the type of environment.
Predict the structure of an unknown compound given an empirical formula and a combination of spectroscopic data (from mass spectroscopy, UV-vis, IR and NMR spectroscopy).
Practise.
