Monday, May 23, 2022

Organic Chemistry V, Video I – Alkynes – Geometry vs Alkenes

In this video we're going to be learning about a family of hydrocarbons that contain triple carbon to carbon bonds such as the simple molecule that we see here this is the simplest form of a family that we call the alkynes indicated by the suffix y and e and we'll see that this suffix appears .

In the name of this simple molecule here so we can actually derive the name there we see that there are two carbons in the parent chain which means that we identify with the prefix f and the suffix y and e indicates the presence of a triple carbon to carbon bond and so this molecule would be named .

Ethyne now the first distinction that we want to make between alkynes and alkenes is the discussion of whether alkynes like alkenes have the same e zed isomer distinction based on their geometry now we remember the reason why alkenes have .

E z isomerism is if we analyze the axe configuration of each of the carbons in the double bond we see that each of those carbons is a x3 meaning that in addition to the double bond each carbon can also have two additional atoms bonded to it like this so let's .

Get rid of one of the hydrogens and let's say that that hydrogen is a chlorine instead and then let's do the same on this side here so we can remember that because this double bond is not able to rotate the way that single bonded atoms are because the .

Double bond increases the amount of resistance that this bond has to um to rotating therefore each of these chlorines is stuck in its respective position and if the chlorine here was pointed up while the chlorine here was pointed .

Down this would be the e isomer because the two chlorines are on opposite sides however this distinction makes no sense when looking at an alkyne because the axe configuration of each of the carbons in the alkyne is only ax2 .

Which means that in addition to the other carbon of the triple bond each ax2 carbon can only contain one other attachment which is the hydrogen here which cannot be pointed up or pointed down because the geometry of an ax2 atom is always going to have each of the .

Attachments 180 degrees apart so ez isomerism makes no sense when it comes to an alkyne versus an alkene now the good news about this is that it makes alkynes relatively straightforward to name because there isn't a second isomer that we need to worry about .

Based on ez positioning across a double bond because across a triple bond this is impossible therefore the only trick is actually recognizing how many carbons an alkyne has because the geometry of an alkyne always has the atoms across from the triple bond 180 degrees apart which makes counting carbons a little .

Bit tricky if you aren't prepared so if we start here recognizing that this can be carbon one not just because it puts the side chain length or side chain location indicators at the lowest number but it also puts the triple bond in the lowest position as well and that is critical now we can .

Recognize that carbon 2 and carbon 3 are here because even though all of these carbons are in a straight line you need two carbons joined at each end of the triple bond like this and then it becomes a fairly simple matter to count the rest .

Of the longest carbon chain so this would be four five six seven so our parent chain in this case is going to be hept because there are seven carbons and then we see that our triple bond is in position two so the parent chain would be called .

Hept two iron and if we identify the rest of our side chains here we can see that on carbons five and six we have two methyl groups so we would identify dimethyl here then on carbon four we have a single ethyl side chain here and then on carbon one of course we have .

Our alphabetically first side chain a bromo and then immediately after that we have one one dichloro to indicate two chlorines on carbon as well and we can see bromo comes first followed by chloro followed by ethyl followed by .

Methyl so let's go ahead and write all of those side chains in the proper order i almost forgot the numerical prefix dichloro to indicate that there are two there's only one ethyl so no numerical prefix is necessary for ethyl and then we have our .

Alphabetically last side chain the 5 6 dimethyl and then i'm going to have to squeeze the parent chain name in here hept and then with the location number two to indicate the presence of the triple bond y and e now if we want to do one practice example .

In terms of drawing this one we've made easy because the triple bond is actually in location number one so the first thing i'd recommend doing is to draw the triple bond out like this and again don't hesitate to label that carbon 1 is here and carbon 2 is here we see that our parent chain is .

Also 7 carbons long so if we remember that because we have ax2 geometry here the angle between carbon 2 and carbon 3 also needs to be 180 degrees but then the rest of the molecule can look relatively straightforward uh with the typical alkane zigzag that we see here so three four five six let's just double check .

One two three four five six seven and there is our parent chain then on carbon three and four we have an ethyl group so let's draw one ethyl on four and one ethyl on three so that we are good and then on carbons 5 and 6 that we have here we have substituted iodines which we can write .

In like this and lo and behold we see yet again that representing alkynes is always easier than actually naming an alkyne from a given structure all you need to do number one is i would recommend identify and draw .

The triple bond location first and remember that because each of the carbons in the triple bond is an ax2 carbon that we need an angle of 180 degrees between all atoms associated with them but other than that the naming and drawing system remains identical .

In the next video we're going to be examining a set of isomers of alkynes only that don't have triple bonds but that actually combine our knowledge of double bonds with our knowledge of drawing ring structures

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