1
00:00:01,968 --> 00:00:03,214
- [Voiceover] Let's
say we're asked to draw

2
00:00:03,214 --> 00:00:04,904
all the structural isomers that have the

3
00:00:04,904 --> 00:00:08,413
molecular formula C5H12.

4
00:00:08,413 --> 00:00:11,233
The word "isomer" means same parts.

5
00:00:11,233 --> 00:00:13,869
And so we're talking about
the same number of atoms.

6
00:00:13,869 --> 00:00:15,664
All of our structural
isomers are gonna have

7
00:00:15,664 --> 00:00:18,709
five carbons and 12 hydrogens.

8
00:00:18,709 --> 00:00:20,243
Our isomers are gonna differ

9
00:00:20,243 --> 00:00:23,138
in how those atoms are
connected to each other.

10
00:00:23,138 --> 00:00:25,866
So they differ in terms
of their structure.

11
00:00:25,866 --> 00:00:28,597
And that's why we call
them structural isomers.

12
00:00:28,597 --> 00:00:31,376
We can also call them
constitutional isomers.

13
00:00:31,376 --> 00:00:33,214
So we need five carbons.

14
00:00:33,214 --> 00:00:35,540
So for our first isomer we could just draw

15
00:00:35,540 --> 00:00:37,380
five carbons in a chain.

16
00:00:37,380 --> 00:00:40,266
So here are my five carbons in a chain.

17
00:00:40,266 --> 00:00:41,643
And you should have already seen the video

18
00:00:41,643 --> 00:00:44,901
on bond line structures
before you watch this one.

19
00:00:44,901 --> 00:00:46,947
So let's draw those five carbons

20
00:00:46,947 --> 00:00:48,471
and let's double check and make sure

21
00:00:48,471 --> 00:00:51,000
we have the correct number of hydrogens.

22
00:00:51,000 --> 00:00:53,909
The carbon on the far
left has three hydrogens,

23
00:00:53,909 --> 00:00:55,945
so here we have our three hydrogens.

24
00:00:55,945 --> 00:00:57,881
Next carbon has two,

25
00:00:57,881 --> 00:00:59,502
same with the next carbons,

26
00:00:59,502 --> 00:01:02,545
so two for this one,
two for the next carbon,

27
00:01:02,545 --> 00:01:07,346
and finally three hydrogens
for the last carbon.

28
00:01:07,346 --> 00:01:08,975
So let's count up everything

29
00:01:08,975 --> 00:01:11,704
and make sure we have to
correct molecular formulas.

30
00:01:11,704 --> 00:01:14,606
We have one, two, three,
four, five carbons.

31
00:01:14,606 --> 00:01:16,137
So that's C5.

32
00:01:16,137 --> 00:01:18,037
And then we should have 12 hydrogens.

33
00:01:18,037 --> 00:01:21,110
Here's three plus two gives us five,

34
00:01:21,110 --> 00:01:23,200
plus two gives us seven,

35
00:01:23,200 --> 00:01:25,668
plus two gives us nine,

36
00:01:25,668 --> 00:01:29,005
and then we have three
more for a total of 12.

37
00:01:29,005 --> 00:01:34,005
So, C5H12 is the molecular
formula for this compound.

38
00:01:34,469 --> 00:01:36,878
Let's draw another structural isomer

39
00:01:36,878 --> 00:01:39,343
that has the same molecular formula.

40
00:01:39,343 --> 00:01:41,445
So instead of drawing
five carbons in a chain

41
00:01:41,445 --> 00:01:43,133
now we have to draw four.

42
00:01:43,133 --> 00:01:45,177
So let's start by drawing four carbons.

43
00:01:45,177 --> 00:01:47,109
We need a total of five carbons

44
00:01:47,109 --> 00:01:48,776
so we need to show the fifth carbon

45
00:01:48,776 --> 00:01:51,104
branching off of our chain.

46
00:01:51,104 --> 00:01:52,667
So we could show the fifth carbon

47
00:01:52,667 --> 00:01:54,976
branching off of our chain here.

48
00:01:54,976 --> 00:01:56,881
Let's draw in those five carbons.

49
00:01:56,881 --> 00:01:59,108
So here we have our five carbons.

50
00:01:59,108 --> 00:02:00,408
Let's count up hydrogens.

51
00:02:00,408 --> 00:02:03,076
Carbon on the left has three,

52
00:02:03,076 --> 00:02:05,208
so three hydrogens here.

53
00:02:05,208 --> 00:02:07,642
Three hydrogens on this top carbon.

54
00:02:07,642 --> 00:02:10,461
There's only one hydrogen on this carbon,

55
00:02:10,461 --> 00:02:12,434
two hydrogens on this one,

56
00:02:12,434 --> 00:02:16,205
and finally three
hydrogens on this carbon.

57
00:02:16,205 --> 00:02:18,281
So let's count up our atoms.

58
00:02:18,281 --> 00:02:19,981
So let's use red for this one.

59
00:02:19,981 --> 00:02:21,776
We have one, two, three

60
00:02:21,776 --> 00:02:23,101
four, five carbons.

61
00:02:23,101 --> 00:02:24,442
So that's C5.

62
00:02:24,442 --> 00:02:26,470
And then for hydrogens we have three here

63
00:02:26,470 --> 00:02:28,435
plus three gives us six,

64
00:02:28,435 --> 00:02:30,103
plus one gives us seven,

65
00:02:30,103 --> 00:02:31,847
plus two gives us nine.

66
00:02:31,847 --> 00:02:34,034
And three more for a total of 12.

67
00:02:34,034 --> 00:02:38,964
So C5H12 is the molecular
formula for this compound.

68
00:02:38,964 --> 00:02:42,104
So these two drawings represent

69
00:02:42,104 --> 00:02:44,549
two different molecules.

70
00:02:44,549 --> 00:02:48,989
Both these molecules have
the molecular formula C5H12.

71
00:02:48,989 --> 00:02:52,646
But they differ in terms of
how those atoms are connected.

72
00:02:52,646 --> 00:02:55,366
They differ in terms of their structure.

73
00:02:55,366 --> 00:02:59,780
So we call them structural
isomers of each other.

74
00:02:59,780 --> 00:03:02,406
All right, to draw
another structural isomer,

75
00:03:02,406 --> 00:03:04,677
some students might say,
"We could start with

76
00:03:04,677 --> 00:03:06,638
"four carbons in our chain again."

77
00:03:06,638 --> 00:03:07,747
And this time,

78
00:03:07,747 --> 00:03:10,709
instead of showing a
branch off of this carbon,

79
00:03:10,709 --> 00:03:13,314
we could show a branch off of this carbon.

80
00:03:13,314 --> 00:03:15,779
And so a student might draw this structure

81
00:03:15,779 --> 00:03:19,070
and say, "Okay, there's a
different structural isomer."

82
00:03:19,070 --> 00:03:21,475
But actually these are
just two different ways

83
00:03:21,475 --> 00:03:24,409
to represent the same molecule.

84
00:03:24,409 --> 00:03:27,672
If you analyze that second
structure that we just drew

85
00:03:27,672 --> 00:03:29,867
the connections are the same.

86
00:03:29,867 --> 00:03:31,770
We have a CH right here

87
00:03:31,770 --> 00:03:35,000
bonded to a CH3, bonded to a CH3,

88
00:03:35,000 --> 00:03:36,839
and bonded to a CH2.

89
00:03:36,839 --> 00:03:39,912
And the CH2 is bonded to a CH3.

90
00:03:39,912 --> 00:03:44,002
That's the same structure as
what we drew out over here.

91
00:03:44,002 --> 00:03:46,403
So it looks like it's
a different structure.

92
00:03:46,403 --> 00:03:48,836
It's a different drawing
than the one up here,

93
00:03:48,836 --> 00:03:51,035
but actually this is
just two different ways

94
00:03:51,035 --> 00:03:54,076
to represent the same molecule.

95
00:03:54,076 --> 00:03:57,072
So we have two structural isomers so far.

96
00:03:57,072 --> 00:03:59,173
Let's think about one more.

97
00:03:59,173 --> 00:04:01,966
So we can no longer do
four carbons in our chain

98
00:04:01,966 --> 00:04:03,947
so we go down to three carbons.

99
00:04:03,947 --> 00:04:06,475
So we start with three
carbons in our chain.

100
00:04:06,475 --> 00:04:08,735
We know we need a total of five carbons.

101
00:04:08,735 --> 00:04:12,700
So we need to show two more
carbons added to our chain.

102
00:04:12,700 --> 00:04:14,709
And these would have to
add those two carbons

103
00:04:14,709 --> 00:04:17,112
to our central carbon like that.

104
00:04:17,112 --> 00:04:20,265
Let's draw out all of our carbons here.

105
00:04:20,265 --> 00:04:22,401
And let's add in our hydrogen.

106
00:04:22,401 --> 00:04:25,146
So this carbon would have three hydrogens

107
00:04:25,146 --> 00:04:27,508
same with this carbon.

108
00:04:27,508 --> 00:04:29,736
And the same with this one,

109
00:04:29,736 --> 00:04:33,435
and finally the same for this carbon.

110
00:04:33,435 --> 00:04:36,679
The carbon in the center, this
carbon in the center here,

111
00:04:36,679 --> 00:04:38,380
already has four bonds.

112
00:04:38,380 --> 00:04:40,870
So it doesn't have any hydrogens on it.

113
00:04:40,870 --> 00:04:42,676
Let's count up everything.

114
00:04:42,676 --> 00:04:43,943
Let's count our carbons first,

115
00:04:43,943 --> 00:04:48,602
one, two, three, four,
five carbons, so C5.

116
00:04:48,602 --> 00:04:50,810
And then we have three hydrogens

117
00:04:50,810 --> 00:04:53,745
plus three is six plus three is nine

118
00:04:53,745 --> 00:04:55,303
plus three is 12.

119
00:04:55,303 --> 00:04:58,407
So C5H12 is the molecular formula

120
00:04:58,407 --> 00:05:00,311
for this compound.

121
00:05:00,311 --> 00:05:03,197
And this is another structural isomer.

122
00:05:03,197 --> 00:05:06,067
So it's a different
molecule from the other two.

123
00:05:06,067 --> 00:05:08,780
So we have a total of
three structural isomers

124
00:05:08,780 --> 00:05:12,807
that have the molecular formula C5H12.

125
00:05:14,366 --> 00:05:16,405
Now let's draw all of
the structural isomers

126
00:05:16,405 --> 00:05:19,947
that have the molecular formula C3H8O.

127
00:05:19,947 --> 00:05:21,340
And we'll start with the molecule

128
00:05:21,340 --> 00:05:23,674
we talked about in the
bond line structure video,

129
00:05:23,674 --> 00:05:25,772
so that molecule look like this.

130
00:05:25,772 --> 00:05:27,905
We have three carbons
and then we have an OH

131
00:05:27,905 --> 00:05:30,851
coming off of the central carbon.

132
00:05:30,851 --> 00:05:32,613
Let's expand that out and make sure

133
00:05:32,613 --> 00:05:35,110
that this has the correct
molecular formula.

134
00:05:35,110 --> 00:05:37,235
We have our three carbons.

135
00:05:37,235 --> 00:05:39,909
And on the middle carbon we have an OH.

136
00:05:39,909 --> 00:05:42,546
So an oxygen bonded to a hydrogen.

137
00:05:42,546 --> 00:05:44,012
I'll go ahead and put lone pairs of

138
00:05:44,012 --> 00:05:46,710
electrons on this oxygen.

139
00:05:46,710 --> 00:05:48,539
How many hydrogens do we need to add

140
00:05:48,539 --> 00:05:50,673
to the carbon on the left?

141
00:05:50,673 --> 00:05:52,108
Well, we need to add three hydrogen.

142
00:05:52,108 --> 00:05:54,404
So we go ahead and draw
in those three hydrogens.

143
00:05:54,404 --> 00:05:56,841
The carbon in the center
already has three bonds

144
00:05:56,841 --> 00:05:59,143
so it needs one more
so we add one hydrogen

145
00:05:59,143 --> 00:06:00,503
to that carbon.

146
00:06:00,503 --> 00:06:02,898
And the carbon on the right
needs three hydrogens.

147
00:06:02,898 --> 00:06:04,997
So let's count everything up now.

148
00:06:04,997 --> 00:06:06,706
So we'll start with our carbons.

149
00:06:06,706 --> 00:06:08,873
We have one, two, three carbons.

150
00:06:08,873 --> 00:06:10,447
So that's C3.

151
00:06:10,447 --> 00:06:13,679
We have three hydrogens
here and three here,

152
00:06:13,679 --> 00:06:16,000
so that's six plus one is seven,

153
00:06:16,000 --> 00:06:19,214
and don't forget about the
hydrogen on the oxygen for eight.

154
00:06:19,214 --> 00:06:20,841
So we have eight hydrogens.

155
00:06:20,841 --> 00:06:23,379
And obviously we have one oxygen here.

156
00:06:23,379 --> 00:06:24,642
So I went ahead and

157
00:06:24,642 --> 00:06:27,076
put in lone pairs of
electrons on that oxygen.

158
00:06:27,076 --> 00:06:29,839
So the molecular formula for this molecule

159
00:06:29,839 --> 00:06:32,833
is C3H8O.

160
00:06:32,833 --> 00:06:35,334
And if I number this, if
I said this was carbon 1

161
00:06:35,334 --> 00:06:38,546
and this was carbon 2,
and this was carbon 3,

162
00:06:38,546 --> 00:06:41,875
that helps us to draw the
next structural isomer

163
00:06:41,875 --> 00:06:42,974
because we could think about

164
00:06:42,974 --> 00:06:45,846
instead of that OH group
coming off of carbon 2,

165
00:06:45,846 --> 00:06:49,044
what if that OH group
came off of carbon 1?

166
00:06:49,044 --> 00:06:51,366
And so let's draw out
our three carbons here.

167
00:06:51,366 --> 00:06:55,600
And now we put our OH group
coming off of carbon 1.

168
00:06:55,600 --> 00:06:57,514
And let's expand this out

169
00:06:57,514 --> 00:06:59,401
and draw the Lewis dot structure

170
00:06:59,401 --> 00:07:00,380
and make sure that this has

171
00:07:00,380 --> 00:07:02,013
the correct molecular formula.

172
00:07:02,013 --> 00:07:05,047
So we have three carbons, again, in a row.

173
00:07:05,047 --> 00:07:06,469
And then the carbon on the left

174
00:07:06,469 --> 00:07:08,046
is bonded to the oxygen.

175
00:07:08,046 --> 00:07:09,933
The oxygen is bonded to a hydrogen.

176
00:07:09,933 --> 00:07:12,975
I'll put in lone pairs of
electrons on the oxygen.

177
00:07:12,975 --> 00:07:15,675
Now we need to add in
carbon hydrogen bonds.

178
00:07:15,675 --> 00:07:17,967
So this carbon needs two.

179
00:07:17,967 --> 00:07:20,010
The next carbon also needs two.

180
00:07:20,010 --> 00:07:22,834
And the carbon on the
end would need three.

181
00:07:22,834 --> 00:07:26,204
So that's one, two, and three.

182
00:07:26,204 --> 00:07:27,607
When we add everything up

183
00:07:27,607 --> 00:07:29,011
let's use blue for that,

184
00:07:29,011 --> 00:07:30,900
that's one, two, three carbons.

185
00:07:30,900 --> 00:07:32,148
We have C3.

186
00:07:32,148 --> 00:07:36,008
We have three hydrogens
here, plus two is five,

187
00:07:36,008 --> 00:07:38,799
plus two is seven, and one here is eight.

188
00:07:38,799 --> 00:07:40,873
So C3H8.

189
00:07:40,873 --> 00:07:42,534
And then, of course, our oxygen.

190
00:07:42,534 --> 00:07:46,241
So C3H8O is the molecular formula.

191
00:07:46,241 --> 00:07:47,348
Next.

192
00:07:47,348 --> 00:07:49,497
Some students might think, "Okay, well,

193
00:07:49,497 --> 00:07:51,864
"we put an OH coming off of carbon 1

194
00:07:51,864 --> 00:07:54,765
"but what if I put an
OH on the other side?"

195
00:07:54,765 --> 00:07:56,637
So, over here on the other side.

196
00:07:56,637 --> 00:07:58,145
So let's see what would that give us.

197
00:07:58,145 --> 00:08:00,734
If I put an OH coming off of that carbon,

198
00:08:00,734 --> 00:08:02,927
hopefully it's obvious that these two

199
00:08:02,927 --> 00:08:05,133
represent the same molecule.

200
00:08:05,133 --> 00:08:06,677
There's no difference in terms of

201
00:08:06,677 --> 00:08:09,300
how those two are connected structurally.

202
00:08:09,300 --> 00:08:10,843
So this is the same molecule,

203
00:08:10,843 --> 00:08:13,170
so two different ways
to draw the same one.

204
00:08:13,170 --> 00:08:15,571
So this is not a new structural isomer.

205
00:08:15,571 --> 00:08:18,949
Just a new way of
looking at this molecule.

206
00:08:18,949 --> 00:08:20,609
Now let's draw one more.

207
00:08:20,609 --> 00:08:23,512
So we can't put the OH
on the other carbon.

208
00:08:23,512 --> 00:08:26,179
So now we have to figure out
something else that we can do.

209
00:08:26,179 --> 00:08:27,644
Well, we could, this time,

210
00:08:27,644 --> 00:08:31,544
put two carbons in a row and
put an oxygen in between,

211
00:08:31,544 --> 00:08:34,903
so putting an oxygen to
break up our carbon chain.

212
00:08:34,903 --> 00:08:37,613
So now this would be
carbon bonded to carbon

213
00:08:37,613 --> 00:08:39,967
bonded to oxygen, bonded to carbon.

214
00:08:39,967 --> 00:08:41,933
And then we fill in our hydrogen,

215
00:08:41,933 --> 00:08:44,747
so there would be three on this carbon.

216
00:08:44,747 --> 00:08:47,377
There would be two on this carbon.

217
00:08:47,377 --> 00:08:49,998
There would be three on this carbon.

218
00:08:49,998 --> 00:08:52,274
And I could put in lone pairs of electrons

219
00:08:52,274 --> 00:08:55,172
on the oxygen like that,

220
00:08:55,172 --> 00:08:56,540
and can everything up.

221
00:08:56,540 --> 00:08:58,942
So we have one, two, three carbons,

222
00:08:58,942 --> 00:09:00,533
so that's C3.

223
00:09:00,533 --> 00:09:04,173
We have three hydrogens plus two is five,

224
00:09:04,173 --> 00:09:05,808
plus three is eight.

225
00:09:05,808 --> 00:09:07,133
So we have the H8.

226
00:09:07,133 --> 00:09:09,043
And then, of course, the one oxygen.

227
00:09:09,043 --> 00:09:12,178
So this is another structural isomer.

228
00:09:12,178 --> 00:09:14,034
Again, some students might say,

229
00:09:14,034 --> 00:09:15,366
"Well, we could go like this,"

230
00:09:15,366 --> 00:09:18,208
and this would be yet another
structural isomer like that.

231
00:09:18,208 --> 00:09:21,410
But really this is just another
way to draw this molecule.

232
00:09:21,410 --> 00:09:23,166
So it's not a new structural isomer.

233
00:09:23,166 --> 00:09:24,907
It has the same connections.

234
00:09:24,907 --> 00:09:27,739
So we have a total of
three structural isomers

235
00:09:27,739 --> 00:09:31,768
that have the molecular formula C3H8O.

236
00:09:31,768 --> 00:09:33,946
And as you go further in organic chemistry

237
00:09:33,946 --> 00:09:37,036
you'll learn that the first
two isomers we talked about,

238
00:09:37,036 --> 00:09:39,110
so this one and this one,

239
00:09:39,110 --> 00:09:40,577
the ones that have an OH on it,

240
00:09:40,577 --> 00:09:42,104
those are called alcohols.

241
00:09:42,104 --> 00:09:45,740
And the last structural
isomer is called an ether.

242
00:09:45,740 --> 00:09:49,508
So we'll worry about that
more later in other videos.