Homework Lab 1
Review of CHM 2010
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Instructor comments: One of the things I want you to do is to learn to look at the chemicals that are reacting
and analyze them in the context of the reaction. What does that mean? The structures of compounds determine
their properties. Reactivity is a property. So, if you analyze the structure, it will reveal how the chemical reacts.
You must also determine the conditions under which the reaction occurs, because a chemical that will react
one way under a certain set of conditions will react another way if the conditions change. What are the conditions?
Is the reagent an acid, base or neutral compound? Is the solvent organic or inorganic (water)? Once you learn to
analyze compounds, you will not have to memorize every reaction in the book. You can reason them out. My
objective is to make you into more of a reasoner and less of a memorizer. The following set of problems is a
review of some first-semester reactions, with some hints to help you reason.
1. Encircle the covalent compounds amoung the compounds shown below.
CH3Cl
HCN
NaCN
NaBr
CH3CN
KCl
[Hint: If a comound contains a metal, it is ionic. If it contains all nonmetals, it is covalent. Go back an see
if you circled a compound containing a metal.]
2. Break the following compounds into their ions, showing charges on the cations and anions.
NaHCO3
NaNH2
NaCN
KBr
NaH
[Hint: An ionic compound is made up of positive and negative ions. If a compound contains a metal, that metal
is a cation (has a positive charge), so NaCl is ionic (Na--sodium--is a metal) and its ions are Na+ and Cl-.]
3. Encircle the ion in the following group that is most basic.
Cl-
-CN
HCO3-
-NH
2
H-
[Hint: When comparing two negatively charged ions for basicity, convert each of them into their conjugate
acids by adding H+ to their formulas. The stronger acid has the weaker congugate base. When comparing
a series of ions, the strongest base is the one with the weakest congugate acid.]
Example: Which is the stronger base, Cl- or Br-?
(1) add H+ to formulas: Cl- + H+ = HCl; Br- + H+ = HBr
(2) HBr is a stronger acid than HCl (periodic trends)
(3) Cl- is a stronger base than Br- because HCl is a weaker acid than HBr
4. Identifying organic families is one of the most important things to learn. Families are determined by structure, and
structure determines properties. Encircle the alkyl halides in the following group.
Br
CH3Cl
Br
Br
OH
Cl
CH3CN
Cl
[Hint: Halogens are F, Cl, Br, and I. If the compound does not have F, Cl, Br or I in its formula, it cannot be
an alkyl halide. If the halogen is bonded directly to a benzene ring it is not an alkyl halide; it is an aryl halide.]
5. Put a box around the alkyl group of each of the following alkyl halides.
I
CH3Cl
CH3CH2CH2Cl
CH3CH2Br
Br
[Hint: The alkyl part of an alkyl halide is all of the molecule except the halogen.]
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6. Alkyl halides are involved in many reactions. Show the products of the following reactions.
CH3CH2Br
+
-CN
CH3CH2Br
+
NaCN
[Hint: When an alkyl halide reacts, it normally transfers the alkyl group to the other reactant while
liberating the halogen as a negative ion. Remember NaCN = Na+ and -CN, so the Na+ is just a
nonreacting spectator. The two reactions above are identical. In the top reaction, the spectator ion
is not shown.]
Show the products of the following alkylation reactions.
7.
8.
Br
+
-OCH
3
Br
+
NaOCH3
CH3C C-
+
Br
CH3C CNa +
Br
O
9.
Br
+
O
Na+
10.
O O
CH3CCHCCH3
Na+
O O
CH3CCHCCH3
Br
+
+
Cl
I
+
[Hint: Sodium ion, Na+, does not participate in organic reactions. Therefore, learn to ignore it and
concentrate on its anion. Sodium is necessary to balance charges, because we cannot have a beaker
with all negative ions in it.]
11. Place a star (*) on the most positive carbon atom in each of the following structures.
Cl
CH3Cl
Br
Cl
Br
[Hint: Halogens are electronegative, they pull electron density toward themselves. Thus, the carbon atom
bonded directly to the halogen will become electropositve, and usually the most positive carbon atom in
the structure.]
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12. Put a box
CH3Cl
around each of the heteroatoms present in the following structures.
O
O
CH3OH
O
CH3CNH2
CH3COCH2CH3
H
OH
2N
NO2
[Hint: A heteroatom is neither carbon nor hydrogen, the most common elements in organic compounds.]
Addition reactions might involve regiochemistry (make one constitutional isomer in preference to another) or
stereochemistry (make one stereoisomer in preference to another. Alkenes and alkynes undergo a number
of addition reactions, which involve regiochemistry, stereochemistry or both. The Markovnikov addition of
HCl to an unsymmetrical double bond is an example of regiochemistry; whereas, the addition of bromine to
a symmetrical ring double bond is an example of stereochemistry. Review the reactions of alkenes and alkynes
and answer the following questions.
13. Show the products of the following reactions.
H2
Pt
HCl
H2O
H2SO4 (catalyst)
14.
CH3C CCH3
H2 (excess)
Pt
CH3C CCH3
H2 (excess)
Lindlar catalyst
CH3C CH
H2O (excess)
H2SO4 and HgSO4
15. Show stereochemistry.
H2 (excess)
Pd-C
Br2
CCl4
Br2
H2O
(1) OsO4
(2) NaHSO3 (aq)
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Let's analyze the following reaction.
NaNH2
+
(1) The organic compound is an alkyne (it has a carbon-to-carbon triple bond). It looks funny, so draw out
the structure.
H
H C C C H
H
(2) We learned earlier that Na+ does not participate, so break the NaNH2 into its ions the way you did earlier.
NaNH2 = Na+ and -NH2
(3) Na+ doesn't react, so what is -NH2? It is a strong base, because its conjugate acid is NH3, an extremely weak
acid (its a base).
(4) The strong base must take a proton (H+) from the organic compound. Which H+ does it take? It will take the
most acidic proton, which for hydrocarbons is the one held to a hybrid orbital with the least p character, sp hybrid
over sp3 hybrid.
2 electrons
H
H
H
sp hybrid
H
C C C
-NH
+ NH3
H C C C H
H C C C
2
(most acidic)
H
H
H
strong
sp3 hybrid
base
takes H +
HNH2
(5) Na+ n ow balances the negative charge on carbon.
+
NaNH2
Na+
+ NH3
16. Show the products of the following sequence of reactions.
(1) NaNH2
(2) CH3CH2Br
[Hint: In step (1), analyze the reactants as above and draw the products. In step (2), note that an
alkyl halide is the reactant, and transfer the alkyl group as you did earlier. This problem links two
important concepts, acid-base reaction and alkylation.
17. A radical contains one nonbonded electron in a valance orbital. An carbanion contains two nonbonding electrons
in a valence orbital. A carbocation has an empty valence orbital. Indicate radical, anion or cation beneath each
of the following species.
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A radical is neutral. What does neutral mean? It means that the chemical (radical in this case) contains
the same number of positive charges as it does negative charges. Where do the charges come from?
The positive charges are all of the protons in the species and the negative charges are all of the electrons
in the species. A neutral bromine atom is a radical, because (1) it is electrically neutral and (2) it has a
valence orbital that holds only one electron in it.
The Lewis (electron-dot) structure for bromine atom is shown below. It is element 35 in the periodic table,
so a neutral bromine atom contains 35 protons in its nucleus and 35 electrons in orbitals outside
the nucleus. Seven of the electrons are in the outermost main shell of bromine and they are called valence
electrons. They pair up as much as possible, but one of these valence electrons is by itself. This single
electron in the neutral bromine makes bromine atom a radical. Radicals are very unstable, so two bromine
atoms combine into Br2 to stabilize the species. The bromine atom is neutral and contains a single electron
in a valence orbital. The other three valence orbitals contain two electrons each, but bromine is not an anion
because the total number of electrons (35) exactly equals the total number of protons (35) in the species.
Br bromine radical
When bromine accepts another electron, such as when it reacts with sodium metal to make NaBr, it becomes
the bromide ion. The bromide ion is shown below. It is an anion (has a negative one charge) because it has
36 electrons but only 35 protons. It contains four valence orbitals with two electrons each.
Br bromine anion
When elemental bromine, Br2, breaks apart heterolytically, as shown below, it makes a bromide ion (negative)
and a bromonium ion (positive).
Br Br
Br
+
bromonium ion
Br
bromide ion
When elemental bromine breaks apart homolytically, as shown below, it makes two radicals.
Br Br
Br
+
Br
two bromine radicals
18. Classify each of the following species as radicals, cations, or anions.
Cl
O
O
19. Show the products.
CH3CH2Br
O
O
20. Show the products.
(1) NaOH (aq)
O
Homework, Lab 1
O
(2) CH3CH2Br
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