Alkanes are a family of organic compounds whose molecules consist only of C and H and SINGLE BONDS.

Transcription

1 ALKANES L. Definition Alkanes are a family of organic compounds whose molecules consist only of and and SINGLE BONDS. You can spot an alkane if its condensed structure has this ratio of to n n+ The name of an alkane ALWAYS ends in -ANE. The prefix of the name will reveal the number of carbon atoms. II. Prefixes The table below gives the names and condensed structures for the first 0 straight chain (unbranched) alkanes. meth- = eth- = prop- = and so on ALWAYS, regardless of the compound or family it is in.

2 First 0!? ow many are there? ow high can you count? You can chain togehter TOUSANDS UPON TOUSANDS of atoms. After about 00 (if not fewer), we stop thinking about them as alkanes and start thinking about them like a plastic known as polyethylene...no need to worry about that now. Let's draw the line-bond structures for a couple... n-propane carbons n-octane 8 carbons n-octadecane 8 carbons The n (italized) before the name means "normal" or that the chain is "straight" This "crooked" alkane is also n-octadecane. It is identical to the zig-zag structure above. To get from the zig zag structure to the crooked structure merely requires that the structure OTATE about its - single bonds. TIS AN AND DOES APPEN. More about that later...just remember a "kink" in the chain does not make a new molecule.

3 III. Bonding in Alkanes: sp ybridization Each atom in an alkane is TETAEDAL. This EQUIES YBIDIZATION of the s and p orbitals of (recall the bonding lesson). 09. o EA ATOM AS SIGMA (!) BONDS. MO of each bond made from an sp hybrid AO on and the s AO on. Let's look at our zig zag and crooked alkane again... bond angles are 09. o Every single one of those atoms are sp hybridized. Every single one is tetrahedral in shape. That's why we draw the zig zag structure. It acknowledges the bond angles and shape in a simple way. - single bonds are formed by an sp hybrid AO from each overlapping. overlapped sp hybrid AOs

4 IV. Branched Alkanes It is possible for alkanes to be branched. This means that the longest continuous chain of atoms has a chain of or more atoms attached to it. longest chain is 8 atoms methyl branch longest chain is 8 atoms methyl branches longest chain is 8 atoms ethyl branch 6 Is this a 6 chain with butyl branch? a. Fooled you. It is just like the alkane above. The longest continuous chain is 8 atoms. There is ethyl branch. 7 8 Branches can have branches.

5 V. yclic Alkanes The : ratio in cyclic alkanes is n n ings can be as small as carbons. You might wonder how small rings (,, atoms) can form if the atom wants to be tetrahedral. EELLENT OBSEVATION. Save it for later. They can also be huge, with lots of atoms. " "chair" cyclohexane Just like the open chain alkanes, the cyclic structures want to have 09. o bond angles if possible. The rings bigger than atoms, therefore, are not FLAT in the plane of this paper, but "bent" or "puckered" to conform to the sp orbital directive. A MOST IMPOTANT EAMPLE IS SOWN ABOVE FO TE 6 atom ring called cyclohexane. And yes, the cyclic alkanes can have branches:

6 VI. ISOMES ISOMES have the same number of and atoms, but arranged in a different order. If I asked you to draw the structure of 0 what would you draw? both of these are 0 TESE AE KNOWN AS structural or ONSTITUTIONAL ISOMES. ere's a few constitutional isomers has 8 constitutional isomers. this one has the nickname "isooctane". It's used as a standard to grade the octane rating of gasoline.

7 VII. Naming There are naming rules that direct how to name alkanes or any other organic compound. Starting with naming alkanes is good because the rules you learn for these simple structures will apply to other families of compounds. The rules are set down by an IUPA (international union of pure and applied chemistry) comittee of chemists and have been adopted world wide. The IUPA systematic naming rules are important for avoiding confusion in identifying structures. Older names, before IUPA, were based on the compound's origin or some property, and are therefore rather arbitrary and random. Some compounds are so well known by their common, older name, that we still use them (acetic acid is an example). I am not going to repeat all the rules here. We'll look at some examples and then you can practice and read the rules in our text. First of all, the table at the beginning of the notes shows you how to name straight chain alkanes. It is common practice to add an n- to the beginning of the name to indicate that the alkane is linear (n means normal). The exception is methane, ethane, and propane, which cannot have branched isomers. -methylbutane. ount the atoms in the LONGEST ONTINUOUS AIN. Name that long chain. Identify each branch point. The LOWEST BAN NUMBE is used in the name. Name the branch point by adding an "-yl" ending to the prefix.. Dashes, not spaces, go between the number and the branch name WONG NUMBEING OF LONGEST AIN The branch is NOT at OET NUMBEING OF LONGEST AIN The branch is at

8 ethyloctane Longest straight chain is 8 atoms, octane The branch is on, ethyl methyloctane Be careful. This is NOT a 7 straight chain with an ethyl branch!!! ethyl--methyldecane When naming two or more branches, alphabetize them in the name. Let's name "isooctane",,-trimethylpentane No matter how you count it, the longest continuous chain is atoms, pentane There are methyl branches, of them are on, of them on Put commas between the numbers. When the branches have the same name, use di, tri, tetra, etc prefixes

9 8 6,6-diethyl--methylnonane 9 7 Longest chain is 9 atoms branches, ethyl (diethyl), methyl alphabetize the branch names, ignore the di- prefix when alphabetizing A couple of cyclic examples: cyclopentane ount the carbons in the ring,, pentane add a cyclo- prefix methylcylcopentane With one branch, no number is needed,-dimethylcyclopentane start numbering at a branchpoint and give each branch the lowest number possible.

10 -ethyl--methylcyclopentane Give the branch points numbers based on the alphabet ethyl gets methyl gets -cyclopropyldecane Don't try to name this with the ring as the "parent" name. Make the ring an alkyl branch name, cyclopropyl.

11 VIII. Physical Properties Non-polar insoluble in water and other polar solvents less dense than water Look at the table below and note what happens to the phase state of an alkane as more atoms are added. Name!mp ()!bp ()!density at 0 (g/ml) methane!-8!!-6!!!gas!! ethane!-8!!-88!!!gas!! propane!-90!!-!!!gas!! butane!-8!!-0!!!gas!! pentane!-0!!6!!!0.66!! hexane!-9!!69!!!0.69!! heptane!-90!!98!!!0.68!! octane!-7!!6!!!0.70!! nonane!-!!!!!0.78!! decane!-0!!7!!!0.70!! at about 0 atoms, alkanes are solid at room temperature BANING LOWES TE MELTING AND BOILING POINTS. For an alkane with n atoms, the straight chain will have the higher m.p. and b.p. TIS IS DUE TO VAN DE WAALS INTEMOLEULA FOES OF ATTATION. The linear chains have a larger surface on which a temporary dipole can be created. more "ball" shaped...less surface contact Elongated...more surface contact

12 I. EATIONS Well, alkanes make good SOLVENTS (medium to carry out a reaction in) because they are INET. There's not a functional group on them. Mixtures of them and other hydrocarbons are burned in engines for energy. owever, there are a couple of reactions I want to point out. Oxidation by ombustion n n+ + (n+)/ O no + (n + ) O + heat for open chain alkanes n n + n/ O no + n O + heat for unbranched cyclic alkanes eat is a product of these reactions. Measuring the heat can give you information about - and - bond strength. Measuring the heat of combustion of cyclic alkanes gives information about ring strain (how strained the bonds must be to make a ring structure). More on that later. alogenation + l h# l + l % % Though there are x more o positions then o, the amount of substitution of l at the o position is greater. For bromination... + Br h# Br + Br 97% % The product is almost entirely Br at the o position

13 + = l 6% % x = Br 80% 0% + Despite a 9: o to o position, the o substitution product is preferred. WY? These reactions form what are called free radicals and the results show that the formation of o and o radicals is preferred over o and that Br radicals are more position sensitive than l radicals. ere's part of the mechanism below. The species with a dot on them are free radicals. That means there is an electron that is unpaired. They are VEY reactive. h# $ o = + kj/mol = l $ o = +9 kj/mol = Br $ o values when methane is the alkane + $ o = + kj/mol = l $ o = + kj/mol = Br most stable when all groups are + $ o = - kj/mol = l $ o = -0 kj/mol = Br According to our product mix data above, the radical seems to select the o or o position over o. What happens is the the radical fomed on is more stable when it is at the o > o > o position. The reaction of a halogen radical at a o position is most productive. The Br radical is less reactive than the l radical, thus making it even more selective. In other words, its interaction at the o position is more productive than at either the o or o positions. We'll revisit this reaction when we discuss the properties of free radicals later.

14 . Synthesis of Alkanes by Alkyl halide oupling: Wurtz eaction The oldest form of the Wurtz reaction involves the use of Na metal to mediate the coupling of two alkyl halides. Other metals can be be used to make the alkyl groups couple. The reaction can be used to make larger alkanes from smaller alkyl groups. Because the reaction proceeds through both a free radical and nucleophilic substitution mechanism, it has some severe limitations that you will understand later when we study radicals and substitution reactions. For now, just realize that this is a classic reaction for producing a new - bond between two alkyl halides. - + ' Na -' + Na new - bond Usually, = ' (can you think of a reason why these groups should be the same? Or, maybe a better question to ask is: what would you get for a product if % '?) The goups are saturated hydrocarbons. For reasons you will understand later, the reaction prefers o or o alkyl halides. This reaction is of limited synthetic value since alkanes are readily availble from other abundant resources (like petroleum for example). Some niche, cool, specialized sytheses can be accomplished with the Wurtz reaction, such as the intramolecular reaction below: + Nal + NaBr -bromo--chlorocyclobutane converted to the bicyclic compound bicyclo[..0]butane via the Wurtz reaction Another thing you can appreciate about the Wurtz reaction is this: it is a simple example of a gigantic and important sub-discipline of organic synthesis known as organometallic chemistry. Organometallic chemistry is responsible for modern plastics and the plastics industry in general.