1 Solvents and general information Flashcards
Describe the most common applications of solvents other than those in analytical and preparative chemistry?
Solvents are used in almost all manufacturing processes in a wide variety of applications. Some common applications of solvents are agrochemicals, pharmaceuticals, home & personal care, cosmetics, electronics, protective oils, coatings (including marine coatings), inks & pigments, paint, dispersing agents for water treatment, metal & surface cleaning and cleaning operations, adhesives, and consumer products in general.
The main applications of solvents in agriculture and in cleaning operations.
In agriculture, the main application of solvents is in end-use pesticide formulations. The inert ingredients in formulated pesticide include carriers or diluents and adjuvants. Most liquid formulations (which can be solutions, suspensions, and emulsions) are diluted with water to make a finished spray, but others might use light oils as carriers. Surfactants are most commonly used as adjuvants to alter the dispersal, spreading, and wetting properties of spray droplets. Surfactants that reduce the surface tension enable the droplets to cross the layer of epicuticular waxes.
In cleaning operations, the two main applications for solvents are in industrial reactors and microelectronics. In industrial reactors, steps must be taken to ensure that the system is thoroughly cleaned between batches. In microelectronics, the standard of purity is very high, similar to that of medicines. Many different types of devices can be processed to address numerous contamination challenges, including particulate matter, oxides, and organic, outgassing, and ionic residues. One particular example is to avoid dendritic growth from ionic contamination in circuit boards.
What are the roles of a solvent in a chemical reaction?
The role of solvents is both physical (most important) and chemical. Solvents allow physical contact between interacting molecules (mass transfer = allows for the best reaction medium), efficient mixing and stirring, and ensure phase homogeneity. They also act as thermal buffers to mediate the heat transfer in reactions: dissipate it in exothermic reactions or transfer it from a heat source to a reacting molecule in endothermic processes (heat transfer). Chemically, through solvation of agents, products, and transition states, solvents contribute to both kinetics (reaction rates) and thermodynamics of the process (equilibria)
Solvents can be classified on the basis of their physicochemical parameters. Quote these parameters and give some examples.
• Boiling point: low-boiling (methanol) or high-boiling (octanol) solvents.
• Density: less dense (methanol/octanol) or more dense (DMSO) than water.
• Viscosity: highly temperature dependent. It is a measure of the intermolecular friction exerted when layers of fluid attempt to slide by one another, and measures the fluid’s resistance to flow. The probability of two reactant molecules to collide and, hence, reaction rate, depends on the viscosity. A viscous solvent does not favor mass transfer. Low (methanol/THF) or high (DMSO).
• Dipole moments and dielectric constants measure solvent polarity. Solvents are classified as polar protic (acidic proton into the skeleton of the compound: acetic acid, methanol, ethanol, ammonia)), polar aprotic (ethyl acetate, diethyl ether, THF, DME), and nonpolar (CCl4, benzene, cyclohexane) solvents.
Examples:
a) Methanol: polar protic solvent, low bp (65°C), low density (0.8 g/mL @20°C), low viscosity (0.543 cP @20°C, 0.392 cP @50°C)
b) 1-Octanol: polar protic solvent, high bP (195°C), low density (0.83 g/mL @20°C), medium viscosity (7.36 cP @25°C)
c) Tetrahydrofuran (THF): polar aprotic solvent, low bp (66°C), low density (0.89 g/mL @20°C), low viscosity (0.48 cP @25°C)
d) Dimethyl sulphoxide (DMSO): highly polar aprotic solvent, high bP (189 °C), high density (1.10 g/mL @20°C), high viscosity (2 cP @20°C)
e) Cyclohexane: nonpolar solvent, low bP (80.7°C), low density (0.78 g/mL @20°C), low viscosity (0.97 cP @20°C)
f) Dicloromethane (DCM): nonpolar solvent, low bP (40°C), high density (1.33 g/mL @20°C), low viscosity (0.41 cP @ 25°C)
Which class of solvents ethanol belong to? Why?
Polar protic solvents: contains a hydrogen bond to an oxygen (-OH) and a dissociable proton (H+). Polar protic solvents are miscible with water (hydrophilic).
Which class of solvents cyclohexane belongs to? Why?
Non-polar solvents: electric charges in the molecules of non-polar solvents is evenly distributed; therefore, the molecules have low dielectric constants and small dipole moments. Non-polar solvents are hydrophobic (immiscible with water).
Why is dimethylsulphoxide considered a super dipolar aprotic solvent?
Dipolar aprotic solvents possess a large bond dipole moment (a measure of polarity of a molecule chemical bond). They do not contain OH, SH or NH groups. A subclassification for the solvents with the highest dipole moment are defined as super-dipolar aprotic solvents; the borderline is around 2.3-2.8 D. Dimethylsulphoxide is considered a super-dipolar aprotic solvent due to its high dipole moment, of 3.96 D.
How do protic polar and polar aprotic solvents solvate ions?
Polar protic solvents are capable of hydrogen bonding because they contain at least one hydrogen atom connected directly to an electronegative atom (such as O-H or N-H bonds). They solvate cations and anions effectively. Some polar protic solvents are water, ethanol, methanol, ammonia, acetic acid, and others.
Polar aprotic solvents contain no hydrogen atoms connected directly to an electronegative atom and they are not capable of hydrogen bonding, but they can accept from other molecules. These are acetone, dimethyl sulfoxide, DMF (N,N-dimethylformamide), acetonitrile, HMF (hydroxymethylfurfural), crown ethers and others.
Provide examples of non-covalent electrostatic intermolecular interactions that are established between solute and solvent.
The non-covalent interactions between solute and solvent are highly dependent of the distance between them. These interactions can be: ion-dipole, H-bond, dipole-dipole, ion induced-dipole, dipole-induced dipole and dispersion. The interaction between methanol and water, for instance, is a H-bond interaction that occurs between the H and O atoms of the molecules. Between the methanol and the chloroform, on the other hand, the interaction is dipole-dipole, that occurs between two polar molecules.
How does the dipole moment move from aprotic polar solvents (e.g. THF) to highly dipolar aprotic solvents (e.g. DMSO)?
The dipolar moment in highly aprotic solvents is much bigger than in aprotic polar solvents. The THF, for instance, has a dipole moment of 1.63D while DMSO has a dipole moment of 3.96D, more than double. The borderline between the aprotic polar solvents and the super-dipolar aprotic solvents is the acetone, with a dipole moment of 2.88D.
List the most serious safety issues related to the use of organic solvents.
- Fire/explosion hazard, dependent on solvent flash point (the lowest temperature at which it can vaporize to form an ignitable mixture in air), vapor pressure, and bp.
- Environmental release for water, air, and soil, particularly for hazardous, persistent, bioaccumulative or toxic substances. Can cause tropospheric ozone formation, stratospheric ozone depletion and/or acid rain and has global warming potential. Ecotoxicity, bioaccumulation potential, mobility in soil.
List the most severe health problems related to the use of solvents.
Skin irritation, eye damage, respiratory or skin sensitization, carcinogenicity, reproductive toxicity (reprotoxic) such as teratogenicity or endocrine disruptors, etc.
List a few major environmental concerns related to the use of solvents.
- VOC releasing solvent (+ NOx = tropospheric ozone formation)
- Stratospheric ozone depletion (chlorofluorocarbons)
- Global warming potential (GH effect)
- Acidification and acid rain or acid deposition
- Ecotoxicity, for example through nutrient salt loads, in which N and P compounds contribute to algal blooms and oxygen depletion (anoxic water), and human toxicity
- Mobility in soils
- Hazardous wastes
Sometimes new solvents are introduced as green solvents, but this label is arguable. Give an example among those examined, underlying why the sustainability claim is controversial.
Alternatives to Et2O, THF, tBME like 2-Methyl-tetrahydrofuran claim to have: 3 times higher Grignards reagents concentrations than in THF, lower partition coefficient and higher stability to HCl; BUT oxidation inhibitors are used, and they are degraded in the atmosphere by OH.
Among solvents obtained from renewable feedstocks, which family of solvents cyrene (write its structure) belongs to? Why?
It is a dipolar aprotic solvent. Dipolar aprotic solvents possess a large bond dipole moment (a measure of polarity of a molecule chemical bond) and do not contain OH, SH or NH groups. Cyrene replaces two highly used but environmentally suspect dipolar aprotic solvents: n-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF).
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