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From Wikipedia

Bond length

In molecular geometry, bond length or bond distance is the average distance between nuclei of two bondedatoms in a molecule.

Explanation

Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond is also a shorter bond, however, there are also few exceptions (Ex, H-H and H-O, the latter one has longer and stronger bond). In a bond between two identical atoms half the bond distance is equal to the covalent radius. Bond lengths are measured in molecules by means of X-ray diffraction. A set of two atoms sharing a bond is unique going from one molecule to the next. For example the carbon to hydrogen bond in methane is different from that in methyl chloride. It is however possible to make generalizations when the general structure is the same.

Bond lengths of carbon with other elements

A table with experimental single bonds for carbon to other elements is given below. Bond lengths are given in picometers. By approximation the bond distance between two different atoms is the sum of the individual covalent radii (these are given in the chemical element articles for each element). As a general trend, bond distances decrease across the row in the periodic table and increase down a group. This trend is identical to that of the atomic radius.

Bond lengths in organic compounds

The actual bond length between two atoms in a molecule depends on such factors as the orbital hybridization and the electronic and steric nature of the substituents. The carbon-carbon bond length in diamond is 154 pm which is also the largest bond length that exists for ordinary carbon covalent bonds.

Unusually long bond lengths do exist. In one, tricyclobutabenzene, a bond length of 160 pm is reported. The current record holder is another cyclobutabenzene with length 174 pm based on X-ray crystallography. In this type of compounds the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°.

The existence of a very long C-C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylenedianions although this concerns a 2-electron-4-center bond. This type of bonding has also been observed in dimers of neutral phenalene dimers. The bond lengths of these so-called pancake bonds are up to 305 pm.

Shorter than average carbon carbon bonds distances are also possible, alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond. In benzene all bonds have the same length: 139 pm. In carbon carbon single bonds increased s-character is also notable in the central bond of diacetylene (137 pm) and that of a certain tetrahedrane dimer (144 pm).

In propionitrile the cyano group withdraws electrons also resulting in a reduced bond length (144 pm). Squeezing a CC bond is also possible by application of strain. An unusual organic compound exists called In-Methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a trypticene and a phenyl group. In an in silico experiment a bond distance of 136 pm is estimated for neopentane locked up in fullerene. The smallest theoretical CC single bond obtained in this study is 131 pm for a hypothetical tetrahedrane derivative.

In the same study, it is estimated that for ethane it takes 2.8 kJ/mol to stretch the CC bond by 5 pm from its equilibrium value and only 3.5 kJ/mol to squeeze it by the same amount. On the other hand, stretching and squeezing by 15 pm requires 21.9 and 37.7 kJ/mol.



From Yahoo Answers

Question:HCl is polar covalent molecule, but why does it form ions in water? What causes it to happen? Ionic compounds that can dissociate are because of formation of ion-dipole bonds that produces energy greater than that needed to break the ionic lattice. HCl is covalent, but it dissociates because of polar nature of water? If so what is the exact interaction between molecules that casues it to happen?

Answers:Polar covalent molecules are the trickiest ones of all. Polar covalent molecules can behave like ionic molecules in a polar solvent. The HCl bond is very polar, which means that there is a large partial positive charge on the H and a large partial negative charge on the Cl. it is "almost" an ionic bond. You put a polar covalent molecule that is almost ionic already in a polar solvent and the water pops off the H from the HCl and you have ions. In a non-polar solvent - such as benzene, the benzene cannot pull the H and Cl away from one another (no affinity for the H or Cl because polar and non-polar species generally don't like one another much) so no ions would form! Dipole-dipole interactions make this possible The same things is seen with sulfuric acid. It forms H+ and SO4^-2 ions in solution - so it is also polar covalent but behaves like an ionic species in a polar solvent (like water)

Question:Since hydrochloric acid(HCL) has stong covalent bonds, why does it dissociate so easily into H+ ions and Cl- ions? (I know it's an acid, but it has strong covalent bonding among atoms!)

Answers:the term HCl ---> H+ + Cl- is the most misleading term that someone could learn in highschool level. But first of all VERY GOOD QUESTION! The term that HCl 'dissociates' in water actually doesn't mean that suddenly HCl molecule breaks into H+ and Cl-, because if that is the case one will need to supply enormous amount of energy. What actually happens is that HCl REACTS with water. since water is considered to be strong basic when compared to HCl, so what really happens is HCl + H2O --> [H3O+][Cl-] and this reaction is an exothermal reaction, that's why when you dilute HCl with water the you'll notice that the reaction glass will be slightly warmer. Since in the past time the most common solvent used in chemistry was water, the definition of acid was also used relatively to water (bronsted acidity concept) and in many case people start to leave out the complete reaction equation and shortened it with HCl ---> H+ + Cl- , it's not wrong though as long as you know that it only applies in aqueous solution and that what really happens is reaction between HCl and water and NOT self-breaking covalent bond of HCl in the more modern chemistry time, more organic solvent are involved and that's why the definition of more universal Lewis acidity become more useful, but this is not in the scope of your question. I hope that would help

Question:for the industrial synthesis of isopropyl alcohol (rubbing alcohol) by reaction of water with propene.

Answers:76 kJ

Question:How much energy would be required to break a single C-H bond? A. 3.98 x10^-32 kJ B. 3.54 x10^-22 kJ C. 213 kJ D. None of these

Answers:213kJ/mol/6.02 E23/mole = 35.4 E-23 = 3.54 E-22 KJ answerB

From Youtube

37. Potential Energy Surfaces, Transition State Theory and Reaction Mechanism :Freshman Organic Chemistry (CHEM 125) After discussing the statistical basis of the law of mass action, the lecture turns to developing a framework for understanding reaction rates. A potential energy surface that associates energy with polyatomic geometry can be realized physically for a linear, triatomic system, but it is more practical to use collective energies for starting material, transition state, and product, together with Eyring theory, to predict rates. Free-radical chain halogenation provides examples of predicting reaction equilibria and rates from bond dissociation energies. The lecture concludes with a summary of the semester's topics from the perspective of physical-organic chemistry. Complete course materials are available at the Open Yale Courses website: open.yale.edu This course was recorded in Fall 2008.