Lecture 3: Polarity Flashcards
What does a molecule require to be polar?
A dipole moment and an assymetrical structure
What is a dipole moment?
The sum of individual bond polarities and lone pair contributions within the molecule
What are polar molecules?
Hydrophillic: Water-loving and Fat-hating
What do polar molecules have?
high melting and boiling points
Why do polar molecules have high melting and boiling points?
Due to the dipole-dipole interactions, which are strong bonds that require a large amount of energy to break
What is an example of a dipole-dipole interaction?
CH3CL – CH3CL: Methyl Chloride
- Chlorine is electro-ve, so all electrons surround the chlorine mol.
- Chlorine has a permanent negative charge/dipole
-Methyl group has a permanent positive charge/dipole as it’s deprived of electron
- When +ve methyl group contacts a -ve chlorine group, produces a dipole-dipole interaction.
How does a dipole moment affect the melting and boiling points of a molecule?
Stronger interaction between molecules = Greater dipole moment.
Greater dipole moment = More energy required to break bonds
What are hydrogen bonds?
Occur between hydrogen molecules and oxygen,nitrogen and fluorine groups.
Why are hydrogen bonds so strong?
Hydrogen molecules are very strong and have no electrons protecting their nucleus.
Therefore the bonds form very close to the nucleus, producing high boiling and melting points.
What is an example of a hydrogen bond?
2 water molecules: H2O- H2O
- Oxygen is electronegative, so electrons surround it
- Oxygen therefore has a permanent negative charge/dipole
-Hydrogen has a permanent positive charge/dipole as they are deprived of electrons
- A hydrogen bond then forms between the negative oxygen and the positive hydrogen
What are non-polar molecules?
Hydrophobic: Water-hating and fat-loving
What do non-polar molecules dissolve and extract in?
non-polar solvents
Why do non-polar molecules have lower boiling and melting points than polar molecules?
The forces between molecules are London dispersion forces/Van der whaal’s forces
These forces are weak and easily broken
Why must samples be extracted?
Samples are composed of an analyte of interest and a matrix.
The analyte must be extracted into a solvent to be analysed
How is extraction used to produce pure samples?
Extraction is used to remove contaminants from a sample.
*Substances must be pure to be analysed
How is extraction used to increase concentration?
Crime scene samples may be so small they’re under the limit of detection.
Extraction can increase concentration
What are the issues with extraction?
- A sample with a large no. of analytes - can be difficult to extract one
- Small sample: Sample can be lost
- Chemical similarities between samples: Difficult to extract one
What are the methods of extraction?
- Liquid-liquid
- Liquid-solid
- Solid phase
- Solid phase micro extraction
What is liquid - solid extraction?
Extracting a solid powder into a liquid e.g. extracting cocaine into a non-polar solvent
What is liquid-liquid extraction?
A liquid sample is partitioned into another liquid sample.
Using a separating funnel containing an aqueous layer and a non-polar layer
What is the method for liquid-liquid extraction?
- Add sample to funnel
- Remove the funnel from ‘O’ ring and shake
- Open valve to release pressure
- Place back in stand & allow layers to equillibriate
- Remove seperate layers into 2 conical flasks
- Evaporate to dryness under nitrogen
What is liquid-liquid extraction dependant on?
- The polarity of the analyte: Polar extracts into polar solvents. non-polar extracts into non-polar solvents
- pH: Specific pH used to analyse acidic, alkali and neutral drugs
What does log P determine?
How polar or non-polar a drug is
What does log P involve?
Liquid-liquid extraction using oil and water
-Hydrophobic drugs extract into the oil
-Hydrophillic drugs extract into the water
What does a negative log P value indicate?
Polar substances, extract into the aqueous layer
What process does log P determine?
How easily drugs can pass through the lipid membrane and reach the site of action
Where are fat soluble drugs stored?
In our fat stores and therefore are rereleased periodically and effects are felt again.
Also pass through the blood-sweat barrier so are deposited in our sweat.
What are acidic drugs composed of?
Acidic functional groups
e.g. carboxylic acid groups
What are basic drugs composed of?
Basic functional groups
e.g. amines
What are neutral drugs?
electrically neutral
What are amphoteric drugs composed of?
basic and acidic functional groups
What are ionised drugs?
soluble in polar solvents
What are non-ionised drugs?
soluble in non-polar solvents
What is an example of an acidic drug?
aspirin due to the carboxylic acid group.
How does aspirin form an acidic solution when dissolved in water?
Becomes deprotonated and loses a hydrogen molecule.
The proton binds to the water molecules, forming H3O+ ions
How are acidic drugs extracted?
Acidic drugs must be extracted at 2PH units below the PKA
e.g. the PKA of aspirin is 3.48 so must extract at a PH of 1.48
What is an example of a basic drug?
Amphetamine due to NH2 Amine group
How does amphetamine form a basic solution in water?
NH2 group becomes protonated forms NH3- ions
Water molecule loses a proton, forms OH- ions
How are basic drugs extracted?
Basic drugs must be extracted at 2 pH units above the PKA
e.g. amphetamine pka = 9.8, must be extracted at pH 11.8
What’s an example of an amphoteric drug?
Morphine as it has acidic and basic functionalities
How are amphoteric drugs extracted?
Amphoteric drugs have 2 pka units due to the 2 functionalities
Amphoteric drugs are extracted at a pH that’s the average of the 2 pkas.