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Forensic Chemistry > Lecture 3: Polarity > Flashcards

Lecture 3: Polarity Flashcards

1
Q

What does a molecule require to be polar?

A

A dipole moment and an assymetrical structure

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2
Q

What is a dipole moment?

A

The sum of individual bond polarities and lone pair contributions within the molecule

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3
Q

What are polar molecules?

A

Hydrophillic: Water-loving and Fat-hating

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4
Q

What do polar molecules have?

A

high melting and boiling points

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5
Q

Why do polar molecules have high melting and boiling points?

A

Due to the dipole-dipole interactions, which are strong bonds that require a large amount of energy to break

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6
Q

What is an example of a dipole-dipole interaction?

A

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.

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7
Q

How does a dipole moment affect the melting and boiling points of a molecule?

A

Stronger interaction between molecules = Greater dipole moment.

Greater dipole moment = More energy required to break bonds

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8
Q

What are hydrogen bonds?

A

Occur between hydrogen molecules and oxygen,nitrogen and fluorine groups.

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9
Q

Why are hydrogen bonds so strong?

A

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.

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10
Q

What is an example of a hydrogen bond?

A

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

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11
Q

What are non-polar molecules?

A

Hydrophobic: Water-hating and fat-loving

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12
Q

What do non-polar molecules dissolve and extract in?

A

non-polar solvents

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13
Q

Why do non-polar molecules have lower boiling and melting points than polar molecules?

A

The forces between molecules are London dispersion forces/Van der whaal’s forces

These forces are weak and easily broken

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14
Q

Why must samples be extracted?

A

Samples are composed of an analyte of interest and a matrix.
The analyte must be extracted into a solvent to be analysed

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15
Q

How is extraction used to produce pure samples?

A

Extraction is used to remove contaminants from a sample.
*Substances must be pure to be analysed

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16
Q

How is extraction used to increase concentration?

A

Crime scene samples may be so small they’re under the limit of detection.
Extraction can increase concentration

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17
Q

What are the issues with extraction?

A
  1. A sample with a large no. of analytes - can be difficult to extract one
  2. Small sample: Sample can be lost
  3. Chemical similarities between samples: Difficult to extract one
18
Q

What are the methods of extraction?

A
  1. Liquid-liquid
  2. Liquid-solid
  3. Solid phase
  4. Solid phase micro extraction
19
Q

What is liquid - solid extraction?

A

Extracting a solid powder into a liquid e.g. extracting cocaine into a non-polar solvent

20
Q

What is liquid-liquid extraction?

A

A liquid sample is partitioned into another liquid sample.
Using a separating funnel containing an aqueous layer and a non-polar layer

21
Q

What is the method for liquid-liquid extraction?

A
  • 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
22
Q

What is liquid-liquid extraction dependant on?

A
  • 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
23
Q

What does log P determine?

A

How polar or non-polar a drug is

24
Q

What does log P involve?

A

Liquid-liquid extraction using oil and water
-Hydrophobic drugs extract into the oil
-Hydrophillic drugs extract into the water

25
Q

What does a negative log P value indicate?

A

Polar substances, extract into the aqueous layer

26
Q

What process does log P determine?

A

How easily drugs can pass through the lipid membrane and reach the site of action

27
Q

Where are fat soluble drugs stored?

A

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.

28
Q

What are acidic drugs composed of?

A

Acidic functional groups
e.g. carboxylic acid groups

29
Q

What are basic drugs composed of?

A

Basic functional groups
e.g. amines

30
Q

What are neutral drugs?

A

electrically neutral

31
Q

What are amphoteric drugs composed of?

A

basic and acidic functional groups

32
Q

What are ionised drugs?

A

soluble in polar solvents

33
Q

What are non-ionised drugs?

A

soluble in non-polar solvents

34
Q

What is an example of an acidic drug?

A

aspirin due to the carboxylic acid group.

35
Q

How does aspirin form an acidic solution when dissolved in water?

A

Becomes deprotonated and loses a hydrogen molecule.
The proton binds to the water molecules, forming H3O+ ions

36
Q

How are acidic drugs extracted?

A

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

37
Q

What is an example of a basic drug?

A

Amphetamine due to NH2 Amine group

38
Q

How does amphetamine form a basic solution in water?

A

NH2 group becomes protonated forms NH3- ions
Water molecule loses a proton, forms OH- ions

39
Q

How are basic drugs extracted?

A

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

40
Q

What’s an example of an amphoteric drug?

A

Morphine as it has acidic and basic functionalities

41
Q

How are amphoteric drugs extracted?

A

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.

Forensic Chemistry (7 decks)

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