CH2 groups

The surface tensions for solutions of organic compounds belonging to a homologous series, for example, R(CH2)nX, show certain regularities. Roughly, Traube [145] found that for each additional CH2 group, the concentration required to give a certain surface tension was reduced by a factor of 3. This rule is manifest in Fig. lll-15b the successive curves are displaced by nearly equal intervals of 0.5 on the log C scale. 

If the dependence on temperature as well as on composition is known for a solution, enthalpies and entropies of adsorption may be calculated from the appropriate thermodynamic relationships [82]. Neam and Spaull [147] have, for example, calculated the enthalpies of surface adsorption for a series of straight-chain alcohols. They find an increment in enthalpy of about 1.96 kJ/mol per CH2 group. 

As discussed in Chapter III, the progression in adsoiptivities along a homologous series can be understood in terms of a constant increment of work of adsorption with each additional CH2 group. This is seen in self-assembling monolayers discussed in Section XI-IB. The film pressure r may be calculated from the adsorption isotherm by means of Eq. XI-7 as modified for adsorption from dilute solution ... 

In this relationship. S is alkane solubility, A is the cavity surface area and a is the hydrophobic free energy per unit area. Extensive fitting of this equation [24] yields a value of 88 kJ mol A for the proportionality constant a. This value corresponds to an unfavourable free energy of about 3.6 kJ mol for the transfer of a CH2 group to aqueous solution. 

Essential Dynamics In most applications details of individual MD trajectories are of only minor interest. An illustrative example due to Grubmuller [10] is documented in Figure 3. It describes the dynamics of a polymer chain of 100 CH2 groups. Possible stepsizes for numerical integration are confined... 

Fig. 3. MD simulation of a polymer chain of 100 CH2 groups due to [10], The dynamics of the distance between two CHj-groups ( 12 and 36). The series of plots illustrates the oscillations of the distance at time scales increasing by a zoom factor of 10 at each level.

In a united atomforce field the van der Waals centre of the united atom is usually associated v ilh the position of the heavy (i.e. non-hydrogen) atom. Thus, for a united CH3 or CH2 group the vem der Waals centre would be located at the carbon atom. It would be more accurate to associate the van der Waals centre with a position that was offset slightly from the carbon position, in order to reflect the presence of the hydrogen atoms. Toxvaerd has developed such a model that gives superior performance for alkemes than do the simple united atom models, particularly for simulations at high pressures [Toxvaerd 1990]. In... 

Analysis Since there is one CH2 group next to the nitrogen atom, this might be a carbonyl group in our usual method of amine synthesis (frame 237). 

For benzylthiazoles and phenethylthiazoles, the influence of one and two -CH2- groups has been determined for the 2-position the results are summarized in Table III-48 (250). 

Table 2 3 lists the heats of combustion of several alkanes Unbranched alkanes have slightly higher heats of combustion than their 2 methyl branched isomers but the most important factor is the number of carbons The unbranched alkanes and the 2 methyl branched alkanes constitute two separate homologous senes (see Section 2 9) m which there is a regular increase of about 653 kJ/mol (156 kcal/mol) m the heat of combustion for each additional CH2 group... 

Cycloalkane Number of CH2 groups Heat of combustion per CH2 group... 

Note however that Table 4 3 mcludes two entries for propane The second entry corresponds to the cleavage of a bond to one of the hydrogens of the methylene (CH2) group It requires slightly less energy to break a C—H bond m the methylene group than m the methyl group... 

When a CH2 group is doubly bonded to a ring the prefix methylene is added to the name of the nng... 

In spite of the ene ending to its name polyethylene is much more closely related to alkanes than to alkenes It is simply a long chain of CH2 groups bearing at its ends an alkoxy group (from the initiator) or a carbon-carbon double bond... 

Methylene transfer from lodo methylzinc iodide converts alkenes to cyclopropanes The reaction is a stereo specific syn addition of a CH2 group to the double bond... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Methyl ketones such as 2 butanone m Figure 17 18 are characterized by sharp singlets near 8 2 for the protons of CH3C=0 Similarly the deshieldmg effect of the carbonyl causes the protons of CH2C=0 to appear at lower field (8 2 4) fhan m a CH2 group of an alkane... 

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

Homologous series (Section 2 9) Group of structurally related substances in which successive members differ by a CH2 group... 

Figure 6.13 Illustration of (a) rocking, (b) twisting, (c) scissoring and (d) wagging vibrations in a CH2 group. Also shown are (e) the torsional vibration in ethylene, (f) the ring-breathing vibration in benzene and (g) the inversion, or umbrella, vibration in ammonia...

These groups are found in the various polymers discussed here. Truly unique in its abiUty to interact and promote water solubiUty is the —O—CH2—CH2— group. The interactions of these groups with water and their placement in the polymer stmcture influence the water solubiUty of the polymer and its hydrodynamic volume. 

Perfluoropolyethers with the linear perfluoropropoxy repeat unit have been commercialized (28). They are prepared by the anionic oligomerization of tetrafluorooxetane followed by direct fluorination to remove the acyl fluoride end group as well as to fluorinate the remaining CH2 groups n can vary widely. 

Alternation is usually above 90%. Nearly perfect alternation of isomeric units in a ca 1 1 monomer ratio has been confirmed by infrared spectroscopy. Bands at 733 and 721 cm have an intensity proportional to the concentration of (CH2) groups (n = 4 and <6, respectively) present in a copolymer containing 46 mol % tetrafluoroethylene intensity decreases with increasing concentration of fluorinated monomer. 

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