Chapter 8 - Haloalkanes, Halogenation, and Radical Reactions

• ^^Another typical error in creating arrow-pushing methods is not using enough arrows.^^

  • %%This is frequently caused by failing to keep track of all lone pairs, bonds formed, or bonds des%%troyed during a mechanism step.

  • In other words, if you precisely evaluate the change location of electrons caused by each arrow, missing arrows will become apparent.

  • %%In this example, we compare two arrow-pushing cases, one of which lacks an arrow.%%

    

• There is no arrow in the erroneous scheme to highlight the breakage of the reactant's C—H bond and production of the p bond in the alkene product.

  • When an arrow is absent, the outcome is frequently too many bonds and/or lone pairs on one atom (see the section on hypervalency below, as shown in the image below).

• Another common error when creating arrow-pushing schemes is to increase an atom's valence to more electrons than the atom can handle, a circumstance known as hypervalency.

  • When carbon is present in stable organic compounds, it possesses eight electrons in its valence shell, according to an underlying principle of organic chemistry (the Octet Rule, as shown in the image attached).

• Many of the other most prevalent elements in organic compounds, such as nitrogen, oxygen, and chlorine, follow the Octet Rule in a similar way.

  • There are three typical methods for pupils to draw hypervalent atoms incorrectly:

^^(1) Having too many bonds to an atom,^^

^^(2) overlooking the presence of hydrogens, and^^

^^(3) overlooking the presence of lone pairs.^^

• An arrow is used to represent a possible resonance contribution in the following scenario.

  • Another typical technique for students to generate a hypervalent atom is to overlook the existence of hydrogens that are not expressly indicated.
  • Because not all hydrogens are drawn when utilizing line angle diagrams to create organic chemical compounds, it is typical to forget them during an arrow pushing exercise.
  • %%The two hypothesized resonance contributing structures of an amide anion are shown in the following example.%%

• The arrow on the left is wrong since it indicates the creation of a new bond to a carbon that already has four bonds.

• When both hydrogen bonds are clearly shown, like in the structure to the right, it is evident that there are now five bonds around the designated carbon atom.

• ^^Another typical hypervalent issue is failing to count all lone pairs of electrons^^.

  • The following example illustrates a negatively charged nucleophile erroneously adding to an alkylated ketone's formal positive charge.

  

• This appears to be right since atoms with positive and negative charges are being directly merged, however when bonds and lone pairs of electrons are counted, it is discovered that the oxygen ends up with 10 electrons total.

  • As a result, this is a mistake.
  • The rationale for these restrictions is because considerable concentrations of strong acids and bases cannot coexist in the same medium because they would undergo a proton transfer reaction before anything else in the solution happened.

• The following is an example of a mixed media mistake.

  • Despite the fact that the reaction is done in acidic circumstances, the first equation demonstrates the formation of a strong base (see conditions over the first equilibrium arrows).
  • The three stages that lead to the intermediate are not depicted.

• The arrow pushing is incorrect since a strong base (methoxide) is formed as the leaving group despite the fact that the reaction is done in strong acid.

  • %%The next step in the proper process would be the protonation of the ether oxygen atom.%%