O—H bond

The force-field model for ethanol contains C-O and O—H bond-stretching contributions in ethane thiol these are replaced by C—S and S—H parameters. Similarly, in ethanol there will be angle-bending terms due to C—O—H, C—C—O and H—C—O angles in ethane thiol these will be C—S—H, C—C—S and H—C—S. The torsional contribution will be modified appropriately, as will the van der Waals and electrostatic interactions (both those within the... 

Search the literature for the experimental results for the H—O bond lengths and the H—O—H bond angle, and include a discussion of the comparison in your report. 

One can start building up a list of MM3 parameters by use of the TINKER analyze command. Don t expect to build up the entire set, which occupies about 100 pages in the MM3 user s manual, but do obtain a few representative examples to get an idea of how a parameter set is constr ucted. From previous exercises and projects, you should have input and output geometries for an alkene, an alkane, and water. From these, the object is to determine the stretching and bending parameters for the C—C, C=C, C—H, and O—H bonds. The C—H bond parameters are not the same... 

Hydrolysis of (CH3CHDO)4B in H2O leads to the formation of ethanol retaining the C—D bond formed in the preceding step while forming an O—H bond... 

The S—H bond is less polar than the O—H bond as is clearly seen m the elec trostatic potential maps of Figure 15 7 The decreased polarity of the S—H bond espe cially the decreased positive character of the proton causes hydrogen bonding to be absent m thiols Thus methanethiol (CH3SH) is a gas at room temperature (bp 6°C) whereas methanol (CH3OH) is a liquid (bp 65°C)... 

In spite of S—H bonds being less polar than O—H bonds thiols are stronger acids than alcohols This is largely because S—H bonds are weaker than O—H bonds We have seen that most alcohols have pA s of 16-18 The corresponding value for a thiol IS about 10 The significance of this difference is that a thiol can be quantitatively con verted to its conjugate base (RS ) called an alkanethiolate, by hydroxide Conse quently thiols dissolve m aqueous base... 

Dipole-dipole interactions especially hydrogen bonding are present m amines but absent m alkanes But because nitrogen is less electronegative than oxygen an N—H bond IS less polar than an O—H bond and hydrogen bonding is weaker m amines than m alcohols... 

Chemical properties of isopropyl alcohol are determined by its functional hydroxyl group in the secondary position. Except for the production of acetone, most isopropyl alcohol chemistry involves the introduction of the isopropyl or isopropoxy group into other organic molecules by the breaking of the C—OH or the O—H bond in the isopropyl alcohol molecule. 

Properties. SUica gel (see Eig. 8) is a coherent, rigid, continuous three-dimensional network of spherical particles of coUoidal sUica. Both sUoxane, —Si—O—Si—, and sUanol, —Si—O—H, bonds are present in the gel stmcture. The pores are intercoimected and fUled with water and/or alcohol from the hydrolysis and condensation reactions (40). A hydrogel is a gel in which the pores are filled with water. A xerogel is a gel from which the hquid medium... 

Figure 4 shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the Hquid state. The O—H bond length within the H2O molecule is 0.96 x 10 ° m. Dipole—dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 4 (not to the same scale) shows the measured H-bond distance of 2.76 x 10 ° m or 0.276 nm. 

Fig. 8.1. Representation of transition states for the first stage of acetal hydrolysis, (a) Initial C—O bond breaking (b) concerted mechanism with C—O bond breaking leading O—H bond formation (c) concerted mechanism with proton transfer leading C—O bond breaking (d) initial proton transfer.

Figure 14.18 Comparison of the molecular dimensions of various gaseous molecules having O-F and O-H bonds.

We will now look at how different types of wave functions behave when the O-H bond is stretched. The basis set used in all cases is the aug-cc-pVTZ, and the reference curve is taken as the [8, 8J-CASSCF result, which is slightly larger than a full-valence Cl. As mentioned in Section 4.6, this allows a correct dissociation, and since all the valence electrons are correlated, it will generate a curve close to the full Cl limit. The bond dissociation energy calculated at this level is 122.1 kcaPmol, which is comparable to the experimental value of 125.9 kcal/mol. 

The influence of an ort/io-imidazole substituent on the bond dissociation energy of the O—H bond in phenol was studied by DFT calculations [00IJQ714]. The imidazole ring is twisted with respect to the phenol ring by 59° and causes a decrease of the bond dissociation energy by about -1 kcal/mol with respect to the unsubstituted molecule only. 

Like the carbon atom in methane and the nitrogen atom in methylamine, the oxygen atom in methanol (methyl alcohol) and many other organic molecules can also be described as sp3-hybridized. The C-O-H bond angle in methanol is 108.5°, very close to the 109.5° tetrahedral angle. Two of the four sp3 hybrid... 

Oxaloacetic acid, an important intermediate in food metabolism, has the formula C4H4O5 and contains three C=0 bonds and two O-H bonds. Propose two possible structures. 

Phenol, C6H5OH, is a stronger acid than methanol, CH3OH, even though both contain an O-H bond. Draw the structures of the anions resulting from loss of H+ from phenol and methanol, and use resonance structures to explain the difference in acidity. 

I The region from 4000 to 2500 cm"1 corresponds to absorptions caused by N-H, C-H, and O-H single-bond stretching motions. N—H and O—H bonds absorb in the 3300 to 3600 cm-1 range C-H bond-stretching occurs near 3000 cm"1. 

Alcohols and phenols have nearly the same geometry around the oxygen atom as water. The R-O—H bond angle has an approximately tetrahedral value (109° in methanol, for example), and the oxygen atom is sp3-hybridized. 

We should point out a serious limitation of the bond enthalpies listed in Table 8.4. Whenever the bond involves two different atoms (e.g., O—H) the value listed is approximate rather than exact, because it represents an average taken over two or more different species. Consider, for example, the O—H bond where we find... 

Both of these reactions involve breaking a mole of O—H bonds, yet the experimental values of AH are quite different The bond enthalpy listed in Table 8.4,464 kj/mol, is an average of these two values. 

For a proton, with its +1 charge, to separate from the molecule, the electron density around the oxygen should be as low as possible. This will weaken the O—H bond and favor ionization. The electron density around the oxygen atom is decreased when—... 

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