Alkynes – 3 Patterns
- “Initial Tails” and “Final Heads”
- 3 Ways To Make OH A Better Leaving Group
- A Simple Formula For 7 Important Aldehyde/Ketone Reactions
- Acids (Again!)
- Activating and Deactivating
- Actors In Every Acid Base Reaction
- Addition – Elimination
- Addition Pattern 1 – Carbocations
- Addition pattern 2 – 3 membered rings
- Addition Reactions
- Aldehydes And Ketones – Addition
- Alkene Pattern #3 – The “Concerted” Pathway
- Alkyl Rearrangements
- Alkynes – 3 Patterns
- Alkynes: Deprotonation and SN2
- Aromaticity: Lone Pairs
- Avoid These Resonance Mistakes
- Best Way To Form Amines
- Bulky Bases
- Carbocation Stability
- Carbocation Stability Revisited
- Carboxylic Acids are Acids
- Chair Flips
- Cis and Trans
- Conjugate Addition
- Curved Arrow Refresher
- Curved Arrows
- Determining Aromaticity
- Diels Alder Reaction – 1
- Dipoles: Polar vs. Covalent Bonding
- E2 Reactions
- Electronegativity Is Greed For Electrons
- Electrophilic Aromatic Substitution – Directing Groups
- Elimination Reactions
- Enantiocats and Diastereocats
- Epoxides – Basic and Acidic
- Evaluating Resonance Forms
- Figuring Out The Fischer
- Find That Which Is Hidden
- Formal Charge
- Frost Circles
- Gabriel Synthesis
- Hofmann Elimination
- How Acidity and Basicity Are Related
- How Are These Molecules Related?
- How Stereochemistry matters
- How To Stabilize Negative Charge
- How To Tell Enantiomers From Diastereomers
- Hybridization Shortcut
- Imines and Enamines
- Importance of Stereochemistry
- Intermolecular Forces
- Intro to Resonance
- Ketones on Acid
- Kinetic Thermodynamic
- Making Alcohols Into Good Leaving Groups
- Markovnikov’s rule
- Mechanisms Like Chords
- Mish Mashamine
- More On The E2
- Newman Projections
- Nucleophiles & Electrophiles
- Nucleophilic Aromatic Substitution
- Nucleophilic Aromatic Substitution 2
- Order of Operations!
- Oxidation And Reduction
- Oxidative Cleavage
- Pi Donation
- Pointers on Free Radical Reactions
- Protecting Groups
- Protecting Groups
- Proton Transfer
- Putting it together (1)
- Putting it together (2)
- Putting it together (3)
- Putting the Newman into ACTION
- Reaction Maps
- Recognizing Endo and Exo
- Redraw / Modify
- Robinson Annulation
- Robinson Annulation Mech
- Sigma and Pi Bonding
- SN1 vs SN2
- sn1/sn2 – Putting It Together
- sn1/sn2/e1/e2 – Exceptions
- sn1/sn2/e1/e2 – Nucleophile
- sn1/sn2/e1/e2 – Solvent
- sn1/sn2/e1/e2 – Substrate
- sn1/sn2/e1/e2 – Temperature
- Strong Acid Strong Base
- Strong And Weak Oxidants
- Strong and Weak Reductants
- Stronger Donor Wins
- Sugars (2)
- Synthesis (1) – “What’s Different?”
- Synthesis (2) – What Reactions?
- Synthesis (3) – Figuring Out The Order
- Synthesis Part 1
- Synthesis Study Buddy
- Synthesis: Walkthrough of A Sample Problem
- Synthesis: Working Backwards
- The 4 Actors In Every Acid-Base Reaction
- The Claisen Condensation
- The E1 Reaction
- The Inflection Point
- The Meso Trap
- The Michael Reaction
- The Nucleophile Adds Twice (to the ester)
- The One-Sentence Summary Of Chemistry
- The Second Most Important Carbonyl Mechanism
- The Single Swap Rule
- The SN1 Reaction
- The SN2 Reaction
- The Wittig Reaction
- Three Exam Tips
- Tips On Building Molecular Orbitals
- Top 10 Skills
- Try The Acid-Base Reaction First
- Two Key Reactions of Enolates
- What makes a good leaving group?
- What Makes A Good Nucleophile?
- What to expect in Org 2
- Work Backwards
- Zaitsev’s Rule
Earlier, I said you could divide the reactions of alkenes into 3 big “buckets”: the carbocation pathway, the “3 membered ring” pathway, and the “concerted” pathway.
A lot of those reactions still work with alkynes. Today, let’s just go through the most straightforward reactions of alkynes.
The carbocation pathway
As with alkenes, strong acids such as H-Cl and H-Br add to alkynes in a Markovnikoff fashion – the halogen (chlorine or bromine) ends up attached to the most substituted carbon. Again, just as with alkenes, these reactions go through the formation of a carbocation, followed by trapping of the carbocation with the halide nucleophile.
If a second equivalent of the acid is added, a second Markovnikoff addition will occur, resulting in a “geminal dihalide” – that is, a carbon with two halogens attached to the same carbon.
Addition of H3O(+) also works with alkynes, and goes through a carbocation – but there’s a twist to this. More on this soon.
The “three membered ring” pathway
With alkenes, halogens like Br2, Cl2, and I2 form a 3-membered ring “halonium” ion which undergoes backside attack to give the trans product. There’s no difference between the pathway of alkenes and that of alkynes. We still get the trans product.
If a second equivalent of Br2, Cl2, or I2 is added, we do a second addition to the resulting alkene, giving a tetrahalogenated alkane.
Oxymercuration also works on alkynes, and results in Markovnikoff products, but (like H3O(+)) there’s an extra twist to this reaction too.
The “concerted” pathway.
This category probably shows the most differences between alkynes and alkenes. Of all the reactions in this category, epoxidation (mCPBA), dihydroxylation (OsO4), and cyclopropanation don’t work with alkynes.
The main reaction to think about in this category is hydrogenation. When alkynes are treated with hydrogen gas and Pd/C, the alkyne will be hydrogenated to the alkene, but it doesn’t stop there. The alkene is then hydrogenated to the alkane.
In order to get the hydrogenation to stop after a single addition, we use a “poisoned” catalyst that doesn’t hydrogenated alkenes. The most common version is “Lindlar’s catalyst”. This gives us the “cis” product.
Hydroboration also works with alkynes, but again, this will be talked about in more detail when we go through the reactions of H3O+ and oxymercuration.
Oxidative cleavage also works with alkynes, but it’s much simpler. Whether we use KMnO4 or O3, we always get carboxylic acids, no matter what.
That’s it! Next time we’ll zero in on some of the key reactions of alkynes.
Thanks for reading! James
P.S. Here’ s a summary sheet on the reactions of alkynes that isn’t on my blog