June 19, 2024 Longchang Chemical

What are the methods of separation and purification of organic compounds?

01

Purification methods for solid organic compounds

Solid organic compounds are generally purified by recrystallization.

1. Recrystallization principle: in a certain solvent, the solubility of solid organic compounds with the temperature change has a large change. The saturated solution at higher temperatures while hot and filtered to remove insoluble impurities. Mother liquor cooling, at lower temperatures when the organic compounds to crystal precipitation, filtration, soluble impurities remain in the mother liquor and removed. Recrystallization can be repeated until the melting point of the purified organic compounds remain unchanged.

2.General operation steps:

(1) Make a saturated hot solution (close to the boiling point of the solvent)

(2) thermal filtration (except insoluble impurities)

(3) Crystallization by natural cooling

(4) filtration of crystals and washing

(5) drying (reduced pressure or atmospheric pressure)

(6) Recrystallize the standard several times until the m.p. is constant

02

Methods of separation and purification of liquid organic compounds

Liquid organic compounds are generally separated and purified by distillation method

1. Principle of distillation: according to the boiling point of each component in the liquid mixture and separation.

2. Key: control the temperature rise slowly, collect the fraction of different temperature range.

3. Distillation method classification:

(1) Atmospheric distillation is suitable for substances with low b.p and do not decompose at b.p.

(2) Decompression distillation applies to b.p higher, or in the b.p decomposition of substances.

(3) Water vapor distillation applies to substances that do not react with water and have a high b.p.

(4) Rotary concentration distillation is generally used to concentrate extracts.

03

Separation of (solid/liquid) organics by column chromatography

1. Principle: the mixture is bound to the stationary phase of the column, the mobile phase flows through the stationary phase from top to bottom, driving the separated components downward. With the stationary phase force of the components in the flow process to stay behind, and with the stationary phase force of the components in the flow process in the front.

2. Column chromatography classification:

(1) Adsorption chromatography column: adsorb components on the solid surface of the adsorbent.

(2) Distribution chromatography column: the inert carrier surface is coated with high boiling point liquid, and the separated material is dissolved and distributed between the eluent and the high boiling point liquid.

(3) Gel chromatography column: multi-gel filling, drenching, gel mesh size sieving different sizes of molecules.

04

Thin layer chromatography separation and purification of organic matter

Thin layer chromatography separation and purification is simple and fast. But the separation capacity is not as large as column chromatography, the effect is not as good as column chromatography.

1. Thin layer classification:

(1) adsorption chromatography

(2) Distribution chromatography: distribution chromatography is divided into normal-phase chromatography and reversed-phase chromatography.

a. Normal phase: water-containing adsorbent (stationary phase), weak polarity mobile phase, small polarity of the fast-moving.

b. Reverse phase: passivated adsorbent (stationary phase), strong polarity of the mobile phase, the polarity of the strong mobile.

2. General procedure: Spotting – Expansion – Color development – Calculation of Rf value.

05

Cryogenic separation method

1. Commonly used in the analysis of organic compounds in foodstuffs: organic compounds are mixed with a large number of fats and waxes, which cause difficulties in separation.

2. Separation principle: As the fat and wax in animal and plant tissues can produce precipitation in acetone solution at low temperature, the sample is repeatedly extracted with acetone, and the extract is frozen, filtered and separated from the fat and wax precipitation, while the organic compounds are left in the acetone.

3. The freezing temperature is generally at -70℃.

06

Liquid phase extraction

1. Extraction principle: a group of immiscible solvents dissolved in a component solute. This solute to a certain proportion (concentration ratio) in the two-phase distribution. Distribution ratio in the two phases is called the distribution coefficient. Separated by utilizing different partition coefficients for each substance in the same group of solvents or different partition coefficients for the same substance in different groups of solvents. (Selection of a suitable solvent and repeated distribution)

2. Classification

(1) isotropic primary distribution: a group of isotropic immiscible solvents to add a solute, after equilibrium, weak polarity solvent in the ratio of organic compounds is P; strong polarity solvent in the ratio of Q; obviously P + Q = 1, the concentration of the two phases of the ratio of P/Q.

(2) isotropic multiple partitioning: take several equal volumes of strong polar solvent in a weak polar solvent extracted several times, the weak polar solvent solute content is P n times, strong polar solvent solvent content is 1- P n times.

(3) Unequal volume of a distribution: let the weak polar solvent volume is strong polar volume of α times. According to the definition, after a distribution of weak polar solvent content is αP, strong polar solvent content is Q; weak polar solvent contains organic compounds accounted for the proportion of the total amount of organic compounds is: αP / (Q + αP).

07

Chemical purification method

Chemical purification method is to make impurities and a reagent chemical reaction to get rid of or eliminate interference. Organic compound extracts interfere with the analyzed components to a large extent are fats and pigments. The most commonly used chemical purification method is to remove impurities by treating organic compounds that are stable to acids with acids or by treating organic compounds that are stable to bases with bases.

08

Solid Phase Extraction Technique

1. Principle: According to the difference of the sample in the two phases, that is, in the solid phase and liquid phase distribution coefficient is different, to achieve the separation and purification of organic substances. The mechanism of retention or elution depends on the analyte and the surface active groups of the solid phase, and the molecular forces between the analyte and the liquid phase.

2. Two elution modes:

(1) The analyte is retained if it has a stronger affinity for the solid phase than it does for the biological medium present. It is eluted with an eluent that has a stronger affinity.

(2) The presence of a biological medium with a stronger affinity for the solid phase is eluted directly with a solvent.

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