1.3 MISTAKES TO AVOID 7 H H-C-OH H-C-OH H-C-OH 1.24 1.3 MISTAKES TO AVOID 1.Nev r draw a carbon atom with more than four bonds.This is a big n -no n at oms only have four orbitals;the ore,carbon atoms can form only four bonds (bonds are formed when orbitals of one atom overlap with orbitals of another atom).This is true of all second-row elements,and we discuss this in more detail in the chapter on drawing resonance structures. 2.When drawing a molecule,you should either show all of the H's and all of the C's,or draw a bond-line drawing where the C's and H's are not drawn. You cannot draw the C's without also drawing the H's: C-C-C-C-C Never do this This drawing is no good.Either leave out the C's(which is preferable)or put in the H's: HH HHH C H or H-C-C-C-C-C- 3.When drawing each carbon atom in a zigzag.try to draw all of the bonds as far apart as possible: is better than 4.In bond-line drawings,we do draw any H's that are connected to atoms other than carbon.For example, OH SH N
1.3 MISTAKES TO AVOID 1. Never draw a carbon atom with more than four bonds. This is a big no-no. Carbon atoms only have four orbitals; therefore, carbon atoms can form only four bonds (bonds are formed when orbitals of one atom overlap with orbitals of another atom). This is true of all second-row elements, and we discuss this in more detail in the chapter on drawing resonance structures. 2. When drawing a molecule, you should either show all of the H’s and all of the C’s, or draw a bond-line drawing where the C’s and H’s are not drawn. You cannot draw the C’s without also drawing the H’s: This drawing is no good. Either leave out the C’s (which is preferable) or put in the H’s: 3. When drawing each carbon atom in a zigzag, try to draw all of the bonds as far apart as possible: 4. In bond-line drawings, we do draw any H’s that are connected to atoms other than carbon. For example, OH SH N H is better than CCCCC C C H H H H H H HH H H H H H H H H or CCCCC C C Never do this 1.3 MISTAKES TO AVOID 7 C C C OH OH OH H H H H H 1.24 6753_Klein_01.qxd 5/1/07 5:03 PM Page 7
8 CHAPTER 1 BOND-LINE DRAWINGS 1.4 MORE EXERCISES First,open your textbook and flip through the pages in the second half.Choose any bond-line drawing and make sure that you can say with confidence how many car- bon atoms you see and how many hydrogen atoms are attached to each of those carbon atoms. Now try to look at the following reaction and determine what changes took place Do not worry about how the changes took place.You will understand that later wher you learn the mechanism of the reaction.For now,just focus on explaining what change took place.For the example above,we can say that we added two hydrogen atoms to the molecule (one on either end of the double bond). Consider another example Br In this example.we have eliminated an hand a br to form a double bond.cwe will see later that it is actually H+and Br-that are eliminated,wher we get into the chapters on mechanisms).If you cannot see that an H was eliminated,then you wil need to count the number of hydrogen atoms in the starting material and compare it with the product: Now consider one more example: B -C In this example,we have substituted a bromine with a chlorine. PROBLEMS For each of the following reactions,clearly state what change has ....we have eliminated ..or we hav
1.4 MORE EXERCISES First, open your textbook and flip through the pages in the second half. Choose any bond-line drawing and make sure that you can say with confidence how many carbon atoms you see and how many hydrogen atoms are attached to each of those carbon atoms. Now try to look at the following reaction and determine what changes took place: Do not worry about how the changes took place. You will understand that later when you learn the mechanism of the reaction. For now, just focus on explaining what change took place. For the example above, we can say that we added two hydrogen atoms to the molecule (one on either end of the double bond). Consider another example: In this example, we have eliminated an H and a Br to form a double bond. (We will see later that it is actually H and Br that are eliminated, when we get into the chapters on mechanisms). If you cannot see that an H was eliminated, then you will need to count the number of hydrogen atoms in the starting material and compare it with the product: Now consider one more example: In this example, we have substituted a bromine with a chlorine. PROBLEMS For each of the following reactions, clearly state what change has taken place. In each case your sentence should start with one of the following opening clauses: we have added . . . , we have eliminated . . . , or we have substituted . . . . Br Cl Br H H H H H Br 8 CHAPTER 1 BOND-LINE DRAWINGS 6753_Klein_01.qxd 5/1/07 5:03 PM Page 8
1.4 MORE EXERCISES 9 CI OH 1.25 Answer: Answer 1.27 c Answer 1.28 Answer 120人 Answer:_ 1.30入入 Answer:
1.4 MORE EXERCISES 9 Cl OH 1.25 Answer: HO OH Cl Br Br O O I SH 1.26 Answer: 1.28 Answer: 1.27 Answer: 1.29 Answer: 1.30 Answer: 6753_Klein_01.qxd 5/1/07 5:03 PM Page 9
10 CHAPTER 1 BOND-LINE DRAWINGS 入久 1.31 Answer: 1.32 Answer: 1.5 IDENTIFYING FORMAL CHARGES Formal charges are charges (either positive or negative)that we must often include ncomplete So you must learn how to identify when you need formal charges and how to draw them.If you cannot do this,then you will not be able to draw resonance structures (which we see in the next chapter).and if you can't do that,then you will have a very and what formal we begin by leamning how formal charges.By doing this you will understand what formal charges are.Sohow do we calculate formal charges? When calculating the formal charge on an atom,we first need to know the number of valence electrons the atom is supposed to have.We can get this number from the periodic table.The column of the periodic table that the atom is in will tell are high school chemistry).For example,carbon is in the fourth column,and therefore has four valence electrons.Now you know how to determine how many electrons the atom is supposed to have. Next we look in our drawing and ask how many electrons the atom actally has in the drawing.But how do Let'ssen exmple.Comsidentral carbon atom in the compound below: 可M H3C-C-CH3
10 CHAPTER 1 BOND-LINE DRAWINGS 1.31 Answer: 1.32 Answer: 1.5 IDENTIFYING FORMAL CHARGES Formal charges are charges (either positive or negative) that we must often include on our drawings. They are extremely important. If you don’t draw a formal charge when it is supposed to be drawn, then your drawing will be incomplete (and wrong). So you must learn how to identify when you need formal charges and how to draw them. If you cannot do this, then you will not be able to draw resonance structures (which we see in the next chapter), and if you can’t do that, then you will have a very hard time passing this course. To understand what formal charges are, we begin by learning how to calculate formal charges. By doing this, you will understand what formal charges are. So how do we calculate formal charges? When calculating the formal charge on an atom, we first need to know the number of valence electrons the atom is supposed to have. We can get this number from the periodic table. The column of the periodic table that the atom is in will tell us how many valence electrons there are (valence electrons are the electrons in the valence shell, or the outermost shell of electrons—you probably remember this from high school chemistry). For example, carbon is in the fourth column, and therefore has four valence electrons. Now you know how to determine how many electrons the atom is supposed to have. Next we look in our drawing and ask how many electrons the atom actually has in the drawing. But how do we count this? Let’s see an example. Consider the central carbon atom in the compound below: H3C C CH3 H O H 6753_Klein_01.qxd 5/1/07 5:03 PM Page 10
1.5 IDENTIFYING FORMAL CHARGES 11 Remember that every bond re ents two electrons being shared between two toms.Begin -H H3C·CCH3 A Now count the numbe of electrons immediately surrounding the central carbon atom 芯H H3C.C--CH3 A There are four electrons.This is the number of electrons that the atom actually ha (in this case.four).Since these numbers are the same.the carbon atom has no for- mal charge.This will be the case for most of the atoms in the structures you will eiee cases,there will be a formal charge.Solet's see anexample of an atom that has a formal charge. Consider the oxygen atom in the structure below: 沁： Let's begin by asking how many valence electrons oxygen atoms are supposed to have.Oxygen is in the sixth column of the periodic table,so oxygen should have six valence electrons.Next,we need to look at the oxygen atom in this compound and ask how many v e electrons it actually has.So,we redraw the molecule by split ting up the C-O bond: In addition to the electron on the oxygen from the C-O bond,the oxygen also has three lone pairs.A lone pair is when you have two electrons that are not being used to form a b ond.Lone pairs ar drawn as two dots on an atom,and th le oxygen above has three of these lone pairs.You must remember to count each lone pair as two
Remember that every bond represents two electrons being shared between two atoms. Begin by splitting each bond apart, placing one electron on this atom and one electron on that atom: Now count the number of electrons immediately surrounding the central carbon atom: There are four electrons. This is the number of electrons that the atom actually has. Now we are in a position to compare how many valence electrons the atom is supposed to have (in this case, four) with how many valence electrons it actually has (in this case, four). Since these numbers are the same, the carbon atom has no formal charge. This will be the case for most of the atoms in the structures you will draw in this course. But in some cases, the number of electrons the atom is supposed to have and the number of electrons the atom actually has will be different. In those cases, there will be a formal charge. So let’s see an example of an atom that has a formal charge. Consider the oxygen atom in the structure below: Let’s begin by asking how many valence electrons oxygen atoms are supposed to have. Oxygen is in the sixth column of the periodic table, so oxygen should have six valence electrons. Next, we need to look at the oxygen atom in this compound and ask how many valence electrons it actually has. So, we redraw the molecule by splitting up the C–O bond: In addition to the electron on the oxygen from the C–O bond, the oxygen also has three lone pairs. A lone pair is when you have two electrons that are not being used to form a bond. Lone pairs are drawn as two dots on an atom, and the oxygen above has three of these lone pairs. You must remember to count each lone pair as two O O H3C C CH3 H O H H3C C CH3 H O H 1.5 IDENTIFYING FORMAL CHARGES 11 6753_Klein_01.qxd 5/1/07 5:03 PM Page 11