Polar Hydrocarbons

Neutral hydrocarbons are generally nonpolar molecules. This is to be expected since carbon-carbon and carbon-hydrogen bonds are relatively nonpolar. Resonance effects can alter this picture, however, by redistributing electrons in novel ways. 

Examine electrostatic potential maps for naphthalene, azulene and hexaphenyltriafulvene. 

Identify the most negatively-charged and most positively-charged regions in each molecule. (Ignore the phenyl rings attached to triafulvene.) The dipole moments of these molecules have been measured as 6.3, 0 and 0.8 debyes. Which molecule is responsible for which dipole moment Explain the trend in dipole moments. 

Some derivatives of triafulvene undergo rotation about the carbon-carbon double bond even at room temperature. Given that cis-trans isomerization about double bonds is normally very difficult (see Chapter 7, Problem 1), how would you rationalize this Examine the electrostatic potential map for perpendicular hexaphenyltriafulvene (the rotational transition state).Would polar solvents tend to lower or raise the rotation barrier Explain. 

Electrostatic potential map for azulene shows negatively-charged regions (in red) and positively-charged regions (in blue). 

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The reaction is considerably modified if the so-called emulsion polymerisation technique is used. In this process the reaction mixture contains about 5% soap and a water-soluble initiator system. The monomer, water, initiator, soap and other ingredients are stirred in the reaction vessel. The monomer forms into droplets which are emulsified by some of the soap molecules. Excess soap aggregates into micelles, of about 100 molecules, in which the polar ends of the soap molecules are turned outwards towards the water whilst the non-polar hydrocarbon ends are turned inwards (Figure 2.17). 

Non-polar hydrocarbon-type liquid phases, e.g. paraffin oil(Nujol), squalane, Apiezon L grease and silicone-gum rubber the latter is used for high-temperature work (upper limit 400°C). 

The so-called G-H solvents were introduced by Palit et al.90 they usually consist of 1 1 mixtures of ethylene or propylene glycol and isopropyl or ra-butyl alcohol (with use of the same mixtures in the titrants) the glycol acts as a solvent for polar groups and the alcohol for the non-polar hydrocarbon groups, which explains the solvating power of their mixtures for both salts and soaps, whereas the alcohol chosen is beneficial for wider pH ranges (see Figs. 4.1 and 2). 

Ionophores are necessary since the lipid components of biological membranes tend to be orientated such that their polar groups face the membrane surfaces while the non-polar hydrocarbon portions occupy the membrane interior. The hydrophobic nature of the centre of the membrane thus acts as a barrier to the passage of ions such as sodium or potassium. 

Acid and base extractions from this material have been shown to form spontaneous structures in solution termed coercevates that could easily form the basis for protypical membranes (more of this in Chapter 9). Hydrocarbons with chain lengths C15-C30 (both straight and branched chains) and of course PAHs, predominantly pyrene and fluoranthrene, polar hydrocarbons such as aromatic ketones, alkyl and aryl ketones, nitrogen and sulphur heterocycles and most intriguingly purine and pyrimidine analogues have all been observed from this rich carbonaceous cocktail of compounds. Why ... 

Reactants and reagents can be conveniently loaded into the dry zeolite by adsorption. This can be accomplished by intimately mixing the solid or liquid reactant and the powdered zeolite, by absorption from the gas phase, or by diffusion in a solvent slurry containing the zeolite and dissolved reactant. The choice of solvent for the slurry method is critical. It must be volatile enough to be removable at a pressure and temperature that does not result in evacuation of the reactant or its decomposition. In addition, the reactant must have a greater affinity for the interior of the zeolite than for the slurry solvent itself. The lack of affinity for the interior of the zeolite is an acute problem for non-polar hydrocarbons that lack binding sites for the intrazeolitic cations. The use of fluorocarbons such as perfluorohexane as slurry solvents takes advantage of the fluorophobicity of many hydrocarbons and has alleviated this problem to some extent.29... 

Increasing the pressure forces the molecules closer together, and the intermolecular interactions become more pronounced. Such interactions are not particularly strong because petroleum gas is a non-polar hydrocarbon, explaining why it is a gas at room temperature and pressure. We discuss other ramifications later. 

We know that emulsifier molecules are made of two parts a long non-polar hydrocarbon chain is attached a polar group which as COONa, S03Na, NH2HC1 or NBr. In micelle formation, the... 

Non-polar hydrocarbon chain of the surfactant molecule o Polar head of the surfactant molecule... 

On the other hand, gas chromatographic methods may overestimate the concentration of total petroleum hydrocarbons in a sample due to the detection of nonpetroleum compounds. In addition, cleanup steps do not separate petroleum hydrocarbons perfectly from biogenic material such as plant oils and waxes, which are sometimes extracted from vegetation-rich soil. Silica gel cleanup may help to remove this interference but may also remove some polar hydrocarbons. 

Polar molecules with a large non-polar hydrocarbon part are less polar than polar molecules with a smaller non-polar hydrocarbon part. For example, octanol, CH3CH2CH2CH2CH2CH2CH2CH2OH, is less polar than ethanol, CH3CH2OH. 

Since aldehydes and ketones are polar, they can act as polar solvents. Because of the non-polar hydrocarbon part of their molecules, aldehydes and ketones can also act as solvents for non-polar compounds. For example, 2-propanone (common name acetone) is an important organic solvent in the chemical industry. 

Solubility in water Amides are soluble in water. Their solubility decreases as the non-polar hydrocarbon part of the molecule increases in size. 

Palkhiwala, A.G., lin, Y.H., Perlmutter, D.D., and Olson, D.H. (1999) Liquid phase separation of polar hydrocarbons from light aromatics using zeolites. Adsorption, 5, 399 07. 

There is a gradation in the solubility of alcohols in water. The lower alcohols (methanol, ethanol and propan-l-ol) are miscible with water because they can hydrogen bond with water molecules. Heptan-l-ol and longer chain alcohols are insoluble in water. So, as the chain length increases, the solubility decreases because the long non-polar hydrocarbon part of the molecule masks the polar hydroxyl group. 

Tetrameric [MeLi]4 (1), [EtLi]4 (2) and [t-BuLi]4 (3) are white pyrophoric powders. While methyllithium is soluble only in polar solvents like diethyl ether, the two others are soluble even in non-polar hydrocarbons like hexane. In non-donating solvent the tetrameric aggregation is retained. Each of the four U3 triangles is /ra-capped by a Ca atom above the center of the equilateral metal triangle. Even in the solid-state none of the three tetramers adopts ideal symmetry (Figure 6). 

Long-chain derivatives of pyrazolyl pyridines coordinated to MoO(02)2 as 35 may efficiently replace the conventional Mo(CO)e in the oxidation of several double-bond typologies with TBHP, in non-polar hydrocarbon solvents . The epoxidation of cyclopentene and 2,3-dimethyl-2-butene led to the corresponding epoxides, notoriously sensitive to ring opening, in 100% and 87% yield respectively, in isoctane, with 1 hour reaction time. Recent advances in the epoxidation of a-pinene and other terpenes using the Mo-TBHP system have been reported. ... 

We did not mention polar hydrocarbons in the crash course on organic nomenclature (Box 10.1). This term refers to compounds containing one or more nitrogen, oxygen, or sulfur atoms replacing carbon atoms in ring structures or sometimes attached to them. These... 

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