Introduction to Organic Chemistry Lecture no. 4 Physical properties
Giuseppe Pileio
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Learning Outcomes By the end of this lecture you will be able: • to Understand the different kind of intermolecular forces • to Understand the concept of boiling/melting point and its link with intermolecular forces • to Understand the concept of solubility and its link with intermolecular forces • to Understand different separation techniques 2
Intermolecular forces Forces that operates between molecules
These forces determine the properties of a material There are different kind of intermolecular forces: • ion-ion
• dispersion
• ion-dipole
• hydrogen bonding
• dipole-dipole 3
Hierarchy of forces There is a hierarchical order of importance of those forces: Ion-ion > H bond > ion-dipole > dipole-dipole > dispersion
pure substances
intermolecular force determine boiling and melting point
solutions
intermolecular force determine solubility 4
Ion-ion electrostatic forces It is the electrostatic force (attaction or repulsion) between two charges (ion are charges)
+ - + -+ +- +
NaCl 5
Ion-dipole forces A pair of opposite charges is called dipole Polar molecules have partial charges of opposite sign distributed in the molecules, i.e. the have dipoles
A ion can interact electrostatically with those charges on the dipole 6
Dipole-dipole forces Dipoles can interact within each-other to give rise to a dipole-dipole force
The partially charges atoms that make the dipole can attract each-other if of opposite sign 7
H-bonding forces This is a particular strong kind of dipole-dipole interaction It usually occurs between an electronegative atom and an Hydrogen attached covalently to another electronegative atom
H2O
protein β-sheet
nucleic acid base pair
Hydrogen bond is frequently also seen as another kind of chemical bond 8
Dispersion forces Dispersion or London forces arise from induced dipoles from one molecule to another They can operate between non-polar molecule
As soon as a temporary dipole is formed by any reason, this can induce an (induced) temporary dipole on a neighbor molecule so that the two can attract 9
Boiling and melting points The boiling point is the temperature at which the phase transition between liquid and gas occurs The melting point is the temperature at which the phase transition between solid and liquid occurs Intremolecular forces determine melting and boiling points Weak forces
Low melting/boiling point
Strong forces
High melting/boiling point 10
Solubility Solubility is the property of molecules (solutes) that dissolves in others (solvent) Golden rule: like dissolves like Intermolecular forces determine the solubility. If solute and solvent can interact by any of the described forces they will dissolve into each other to some extent proportional to the strength of the interaction hexane is a non-polar hydrocarbon, water is polar then they do not mix 11
Trend of BP I. Alcohol vs alkane Structure CH3CH2OH CH3CH2CH3
Name ethanol propane
MW 46 44
BP (°C) 78 -42
II. Alcohol vs ether Structure CH3CH2OH CH3OCH3
Name ethanol dimethyl ether
MW 46 46
BP (°C) 78 -24
MW 60 60 58
BP (°C) 118 97 49
III. Acid vs alcohol vs aldehyde Structure Name CH3COOH ethanoic acid CH3CH3CH2OH 1-propanol CH3CH2CHO propanal
• • • •
Alcohols can have H-bonds and is also polar Alkanes are not polar Ethers can have H-bonds but are less polar than alcohols Carboxylic acids are polar and can make 2 H-bonds each BPCOOH > BPOH > BPROR > BPCC
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Trend of Solubility I. Alcohol vs alkane Structure CH3CH2OH CH3CH2CH3
Name ethanol propane
MW 46 44
Sol in H2O infinite -
II. Alcohol vs ether Structure CH3CH2OH CH3OCH3
Name ethanol dimethyl ether
MW 46 46
Sol in H2O infinite 5-10% (by wt.)
III. Acid vs alcohol vs aldehyde Structure Name CH3COOH ethanoic acid CH3CH3CH2OH 1-propanol CH3CH2CHO propanal
MW 60 60 58
Sol in H2O infinite infinite 10-20% (by wt.)
IV. Many groups Structure CH3CH2CH2OH CH3CH2CH2CH2OH CH3CH2CH2CH2CH3OH
MW 60 74 88
Sol in H2O infinite 7.9 g/100ml 2.7 g/100ml
Name propanol butanol pentanol
There is a competition between the insolubility of the alkyl chain and the solubility of the polar groups. These 13 molecules are called Amphiphatic
Separation of substances A process of separation is used to split a mixture of substances into its different components Physical properties of substances are used for the purpose: 1) Separation by boiling point 2) Separation by polarity 3) Separation by affinity 4) … 14
Distillation: Separation by boiling point Distillation is the simpler example of separation on the basis of boiling point The mixture is warmed slowly to a certain temperature just above the first, lowest boiling point, component This component is trapped in another cooled container The mixture is warmed to the temperature of the next lowest boiling point component 15
Chromatography Chromatography is a collection of separation techniques of the same kind: They all involves passing a mixture contained into a mobile phase through a stationary phase which separates certain components using different molecular properties The retention time (time needed to a compound to elute) is determined by the interplay of two factors: 1. Intermolecular forces between analyte and stationary phase 2. Intermolecular forces between eluent and stationary phase 16
Depending on the state of the mobile and the stationary phases there are a number of techniques: Abr.
name
mobile
stationary
GC
Gas Chromatography
gas
solid/liquid
LC
Liquid Chromatography
liquid
solid/liquid
TLC
Thin Layer Chromatography
liquid
solid
Non-Polar phase: Interact with non-polar solutes
Polar phase: Interact with polar solutes
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Gas Chromatography Is a type of chromatography in which the mobile phase is a carrier gas while the stationary phase is a microscopic layer of liquid or polymer on an inert solid support, called column Frequeltly used in toxicology/drug testing Compound must be volatile: • Ideal for small organic molecules • large biomolecules are not sufficiently volatile 18
• A known volume of gaseous or liquid sample is injected • As the carrier gas sweeps the sample through the column, the motion is inhibited by interactions • The rate at which the molecules progress along the column depends on the strength of interaction • A detector is used to monitor the outlet stream
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Liquid Chromatography Is a type of chromatography in which the mobile phase is a liquid while the stationary phase is a microscopic layer of liquid or polymer on an inert solid support, called column HPLC: High Performance (Pressure) Liquid Chromatography, forces the analyte through the column using high pressure The use of pressure increases the speed giving the components less time to diffuse within the column, leading to improved resolution in the resulting chromatogram 20
Chromatography in Biology Separation of amino acids using non-polar phase LC The stationary phase is hydrophobic so hydrophilic amino acids are eluted first since they form weakest interactions with the column
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Chromatography in Biology Analysis of Nucleoside content in DNA samples First, DNA is broken enzimatically into nucleosides
• The 4 nucleosides A, G, C and T are retained in a different way on a nonpolar column • Composition of the sample can be elucidated 22
Chromatography in Biology Separation of enantiomers Ibuprofen is chiral and exists as two enantiomers
• Chiral stationary phase • The two enantiomers have different interactions with a chiral stationary phase and then can be separated
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What we learn in this lecture?
• What are intermolecular forces • Different kind of intermolecular forces and their relative strength • How intermolecular forces influence properties like boiling/melting point and solubility • How to use these properties to separate mixture of substances
