MCAT and Organic Chemistry Study Guides, Videos, Cheat Sheets, tutoring and more

  • Tutoring
  • Organic Chem
    • Organic Chemistry Tutoring
    • Organic Chemistry Videos
    • Study Hall – Membership Site
  • MCAT
  • About
  • Contact
  • Tutoring
  • Organic Chem
    • Organic Chemistry Tutoring
    • Organic Chemistry Videos
    • Study Hall – Membership Site
  • MCAT
  • About
  • Contact

Pencil Trick For Classifying Primary Secondary and Tertiary Carbon Atoms

February 26, 2014 By Leah4sci 74 Comments

pencil trick for identifying degree of carbon atomsAs an organic chemistry tutor, one of the first and more complicated topics that my students need help with is how to quickly recognize primary, secondary and tertiary carbon atoms within a molecule. There are a number of ways to do this.

I’ll explain them then show you a trick you can apply to get your answer quickly on an exam. I call it the pencil trick.

After reading, be sure to watch the step-by-step Video Tutorial at the bottom of this page.

 

Overview of Primary Secondary and Tertiary Atoms

Primary, secondary or tertiary carbon refers to the number of carbons directly attached to the carbon in question.

primary secondary tertiary and quarternary carbon In other words:

  • A primary carbon can be written as 1° (#1 with a degree symbol) has one carbon attached to this carbon atom.
  • A secondary carbon written as 2° (#2 with a degree symbol) is a carbon attached to two other carbons.
  • A tertiary carbon written as 3° (#3 with a degree symbol) is a carbon attached to three other carbons.
  • And a quaternary carbon written as 4° (#4 with a degree symbol) is a carbon attached to four other carbons.

I want you to recognize that the quaternary carbon is attached ONLY to carbon. This is because having four bonds to carbon means that carbon already has its complete octet. But primary, secondary and tertiary carbons will have other atoms attached. These atoms can include hydrogen, halogens (F, Cl, Br, I), oxygen, nitrogen, and so on.

Degree of Substitution

Degree of substitution means how many carbons are attached to the carbon atom in question. The answer would be something like primary, secondary or tertiary depending on what you have. The quickest way to recognize the degree of substitution is to look at the carbon in question and count how many carbon atoms stem from it.

When you have a molecule written in Lewis Structure it’s easy to see but when the molecule is written in line structure it can get tricky.

This is Where The Pencil Trick Comes In To Play

pencil trick for primary and secondary carbon atoms

Here’s how to quickly recognize if a carbon atom is primary, secondary, tertiary or quaternary using the pencil trick. Identify the carbon in question and place your pencil on that carbon atom. Then examine how many lines emanate from the carbon atom that attach to another carbon—meaning the line does not lead to a hydrogen, oxygen or halogen. The number of lines that stem from that carbon represents the degree of substitution. Look back at the image above.
On the image I highlighted one carbon atom in yellow which, if you put your pencil on it, has three lines emanating from it making it a tertiary carbon. If you put your pencil on the other carbon atom, highlighted in green, you’ll see two lines stemming from it, making this a secondary carbon.

Alcohol and Halogen Substitution

pencil trick for secondary halogen

When referring to the degree of substitution of a halogen or an alcohol, you don’t look at the halogen or alcohol itself but rather at the carbon holding it.

So for example, if you look at bromine in the molecule drawn here, you’ll see that bromine is attached to one carbon. But this bromine is not a primary halogen even though it’s attached to one carbon. You must look at the carbon instead. Do the pencil trick and see that this carbon is attached to two other carbons. Since the carbon is secondary, so is the bromine.

pencil trick for tertiary alcohol

The same thing applies for alcohol. For example, if the alcohol is on the tertiary carbon then the alcohol is considered tertiary; not because it’s attached to one carbon but because the carbon holding the alcohol is attached to three other carbons.

Amines (Nitrogen) Are Classified Differently

Degree of substitution for amines is slightly different than alcohols and halogens because in this case we’re actually looking at the nitrogen atom.

A primary amine is one that has the nitrogen or an NH2 group directly attached to one carbon. We don’t care if this is a primary, secondary or tertiary carbon because it’s the nitrogen that we’re examining.

A secondary amine is an NH group attached to two other carbons.

A tertiary amine is a nitrogen attached to three other carbons.

Sometimes you'll see an ammonium salt, which is a positive nitrogen attached to four different carbons.

The reason I said NH2, NH and N for primary, secondary and tertiary is because we’re assuming our amines are neutral. If nitrogen can have a total of three bonds and one lone pair, we expect that when nitrogen is bound to one carbon, it will have two hydrogens.

When nitrogen is attached to two carbons it will have one hydrogen.

When nitrogen is attached to three carbons it will not have hydrogen.

That’s why ammonium is the different one, the positive nitrogen has 4 bonds and no lone pairs.
You can actually apply the pencil trick to nitrogen as well. Just put your pencil on nitrogen and see how many carbons stem from it.

(Watch on YouTube: Pencil Trick. Click cc for transcription.)

Do you see the pencil trick simplifying your life? I'd love to read your feedback in the comments below

Filed Under: Study Tips Blog Tagged With: primary, secondary, tertiary

Comments

  1. Drew says

    August 7, 2019 at 8:22 pm

    Would an alcohol attached to a benzene (phenol) be considered a secondary or tertiary alcohol? The OH is attached to only 2 other carbons, but it has 3 bonds, granted one is a pi bond. So, as with all groups in question theatre attached to an sp2 carbon, are they secondary or tertiary. Similarly, one could ask: is a group that’s attached to an sp carbon (alkyne) be considered a primary or tertiary group?

    Reply
  2. Arpita Baruah says

    May 20, 2019 at 1:36 pm

    The explanation are awesome. Thank you mam.

    Reply
  3. Cole Andrew Sonksen says

    February 3, 2019 at 10:20 pm

    what about a double bond in the ring

    Reply
  4. sarifm says

    November 30, 2018 at 10:23 pm

    Hey Leah on the Morphin example when you are counting the secondary C do we not take account the bonds found within the aromatic ring?

    Reply
  5. Mrunal says

    November 23, 2018 at 9:10 am

    Nice trick……….Thanks

    Reply
  6. Banya says

    August 25, 2018 at 7:27 am

    I loved it..thank you

    Reply
  7. Brandon Benefield says

    May 9, 2018 at 5:28 pm

    whats the difference in CH3 and H3C and why do you use both forms?

    Reply
  8. Ricky says

    April 14, 2018 at 11:39 pm

    If there is a double or triple bond between any two carbons…?? Then….

    Reply
  9. oghenekeno says

    April 9, 2018 at 10:00 pm

    love,love,love it thanks leah!!!

    Reply
  10. Angelica Quow says

    March 8, 2018 at 7:34 am

    THANK YOU !!!!

    Reply
  11. Samir says

    February 18, 2018 at 9:52 am

    Mam
    How do you classify Hydrogen atoms as Primary secondary or tertiary?
    I always get muddled up in that. Any tips?

    Reply
  12. Shahid Rana says

    February 9, 2018 at 1:22 am

    Hi
    I have a question
    Plz tell me how many primary carbon atoms are present in 1-propyne?

    Reply
  13. Edna says

    January 19, 2018 at 7:52 pm

    I love your technique in explaning that topic.
    Hope to learn more from you..

    Reply
  14. Vivek Goel says

    January 10, 2018 at 4:47 pm

    Thanks for the nice, simple yet very effective explanation – it helped me a lot coming quickly to a good understanding of the difference between primary, secondary and tertiary amines and the difference in nomenclature between amines and other functional groups. Thanks much!

    Reply
  15. Neelam says

    December 21, 2017 at 11:01 am

    Ya solved my query quite smoothly……thank. can ya tell tricks for other things too?????

    Reply
  16. Tejaswini says

    November 25, 2017 at 4:04 am

    Thank you.. Helpful trick solving these type of question is now easier with the help of your trick.

    Reply
  17. Kiran Teja says

    November 11, 2017 at 5:53 am

    Thanks Leah, very useful. Pencil trick is excellent. U gave me clarity about primary, secondary, tertiary.

    Reply
  18. Saad Q says

    October 21, 2017 at 9:49 pm

    God bless you 🙂

    Reply
  19. Hannah Davis says

    September 29, 2017 at 12:15 pm

    I don’t understand how you identify the carbon in question, especially if, for instance, you have a kekule structure with a cabon in the center, three CH3’s attached to the center carbon, and NH2 also attached to the center carbon. I want to call this tertiary because the central carbon is attached to three carbons, but it is incorrect and I don’t understand why.

    Reply
  20. Karlz says

    September 15, 2017 at 6:54 pm

    What do you do in the case of a double bond?.. let’s say, in a cyclohexane? How would you count the carbons?

    Reply
  21. Devpriya says

    August 16, 2017 at 10:29 am

    Thank you so much ma’am…..the trick is really helpful.

    Reply
  22. Vijay patel says

    July 2, 2017 at 4:45 am

    Thank u so much , it really very helpful to me . Thanks a lot

    Reply
  23. maurice henson says

    June 11, 2017 at 9:20 pm

    This summer I am prepping for Org 1. I have not had the opportunity to apply your teaching formally. I would rate this particular topic explanation very good. I think you could improve upon the very good topic link by: 1. I spent 3 min studying the first figure, then the wording made sense. I think if you put emphasis/expanded on the initial carbon and the first degree, by elaborating that the carbon itself is “connected” to itself, and itself only, which by default makes it primary, would have helped, similar to how you started your explanation of the quaternary. 2. I think you should have kept your color coding consistent by marking the alcohol with purple instead of orange. 3. I think you should have included a figure in your great teaching of the nitrogen(amine) group. Thanks for creating a great website.

    Reply
  24. Udit says

    May 27, 2017 at 8:48 am

    if there is a double bond between 2 carbons, for example in ethene (H2-C doublebond C-H2)
    Are both the carbons still primary or they are secondry?
    Do bonds matter in defining the primary, sec.,and tert. carbons?

    Reply
  25. Lauren Myers says

    March 30, 2017 at 2:27 pm

    On top of working full time, being a mom, going to school full time, and doing a program at Stanford University twice a month, you give me SO much hope! Freakin’ adore you and this pencil trick! My day just got 100{4de72ea05502f43f582be05109791d04c2e06ba4d55e604218cbe1ea315cd788} better. Binge watching O’Chem videos this weekend! You are loved. Thank you for all you do.

    Reply
  26. Divyesh says

    March 28, 2017 at 10:51 pm

    That was really helpful

    Reply
  27. Dharanya says

    March 6, 2017 at 2:48 am

    It is amazing. Can you explain about hybridisation???

    Reply
  28. Akash Yadav says

    February 28, 2017 at 2:43 pm

    Great trick

    Reply
  29. Emilio says

    February 27, 2017 at 7:04 am

    I want you to know that i’m so grateful for the effort you put in. I have dyslexia and studying medicine and it’s thanks to people like you i can manage the studies

    Reply
  30. jonah says

    January 17, 2017 at 10:03 am

    I still dont get it. But leah4sci girl in pic is so cute

    Reply
  31. janifer says

    December 17, 2016 at 2:12 pm

    success is depend on your education if you have good education with degrees success is not far for you but if you have only experience but not education equal to your experience you never get success for those people online universities offers buy a degree online with transcript equal to your experience.

    Reply
  32. Swagatam Biswas says

    November 10, 2016 at 1:58 am

    Thats really an Amazing trick… It would be great if you give some more examples!

    Reply
  33. Marga says

    November 4, 2016 at 4:26 am

    huh please help i still don’t get it

    Reply
    • Leah4sci says

      December 29, 2016 at 5:22 pm

      Marga, can you be more specific about what you don’t understand?

      Reply
    • Teresia Jones says

      June 23, 2018 at 12:36 pm

      It’s simple…go back and watch it again

      Reply
  34. peacelovetsd says

    October 18, 2016 at 5:04 pm

    Thank you, this is SO helpful! Such a simple idea that really works. Finally something ‘simple’ in orgo! 🙂

    Reply
    • Leah4sci says

      December 29, 2016 at 5:38 pm

      You’re welcome! Glad it clicked for you!

      Reply
  35. armin says

    September 8, 2016 at 8:48 am

    you are really the best

    Reply
    • Leah4sci says

      September 19, 2016 at 1:25 pm

      Thanks, Armin!

      Reply
  36. zoid says

    September 8, 2016 at 1:37 am

    so do you refer to an ammonium salt with 4 carbons as Quaternary?

    Reply
  37. jithendra says

    May 20, 2016 at 4:49 am

    trick looks pretty like you only sister
    itz amazing i love it…………..:)

    Reply
    • Leah4sci says

      May 26, 2016 at 11:02 am

      Jithendra, uh… thanks?

      Reply
  38. Yumna says

    May 15, 2016 at 12:23 pm

    HELPED A LOT! THANK YOU SO MUCH ..

    Reply
    • Leah4sci says

      July 3, 2016 at 11:03 am

      You’re very welcome Yumna

      Reply
  39. jui says

    April 22, 2016 at 10:27 am

    Thx soooo much

    Reply
    • Leah4sci says

      May 3, 2016 at 8:45 am

      You’re welcome Jui

      Reply
  40. Shreya Patel says

    April 13, 2016 at 12:33 pm

    Thank you

    Reply
  41. Leah4sci says

    April 4, 2016 at 5:37 pm

    Thanks Priyanka

    Reply
  42. Naseer Hatim khan says

    March 16, 2016 at 12:46 am

    you are a legend miss
    you are the greatest teacher of my entire life
    respects

    Reply
    • Leah4sci says

      April 4, 2016 at 5:45 pm

      Thanks Naseer

      Reply
  43. nadhrah zulkifli says

    January 26, 2016 at 9:13 am

    do u have any video that explaining above stuff? cause i dont really get it

    Reply
    • Leah4sci says

      January 26, 2016 at 7:35 pm

      Sorry, no

      Reply
  44. Shashank says

    January 21, 2016 at 10:25 am

    This s jst super cool…dis helped me a lottt!!! Guys…..jst follow dis ull…find d soln for ua prob…!! All d bst!!

    Reply
  45. Marissa says

    October 2, 2015 at 6:48 pm

    This makes so much since how you explained it! I watched your other videos for Organic Chem (I’m in O. Chem 1 right now) and it helped me so much when I was studying for my midterm I had the other day. Thank you!

    Reply
    • Leah4sci says

      October 10, 2015 at 5:43 pm

      That’s awesome Marissa! I hope you aced your midterm

      Reply
  46. mike leyden says

    September 16, 2015 at 7:38 am

    Thanks for sharing best tips for using of pencil tricks and i hope you regular give us the great information.

    Reply
    • Leah4sci says

      September 28, 2015 at 6:38 pm

      You’re welcome! I have a TON of resources on the site so be sure to use the search bar to find them

      Reply
  47. Pankaj Sinha says

    March 26, 2015 at 12:44 pm

    Thanks Leah. You are great.

    Reply
    • Leah4sci says

      March 26, 2015 at 10:03 pm

      You’re welcoem Pankaj

      Reply
    • Ruth Casseus says

      May 17, 2015 at 11:05 pm

      thank you Leah. you are great so much easier

      Reply
      • Leah4sci says

        May 21, 2015 at 11:59 pm

        You are very welcome Ruth

        Reply
  48. Russ Hollifield says

    October 27, 2014 at 11:44 pm

    Hey, that’s a really simple but EFFECTIVE way to figure out primary, secondary, and tertiary…thanks Leah!!

    Reply
    • Leah4sci says

      October 28, 2014 at 9:07 am

      Glad you find it helpful Russ. If you’re allowed a highlighter on exams it’s even easier

      Reply
  49. Damarys says

    October 2, 2014 at 11:18 pm

    Very good trick Leah,.. easy to visualize it… thanks!

    Reply
    • Leah4sci says

      October 2, 2014 at 11:20 pm

      Thank you Damarys. I love this trick because it’s so ‘childish’ and fun, yet works at the college level. I’m a kid at heart so… 🙂

      Reply
  50. loren says

    July 30, 2014 at 4:32 pm

    organic is soo easy lit my favourite subject. i dont understand why so many people complain about orgo.

    Reply
    • Leah4sci says

      August 4, 2014 at 11:37 am

      Organic is a fun topic, but not that easy to get compared to the other sciences. Since most students don’t get it right away, and don’t always have the best resources available, they complain…

      Reply
  51. Leah4sci says

    June 8, 2014 at 7:19 pm

    Awesome, glad I can help. What else did you get in addition to the pencil trick?

    Reply
    • Johnson says

      June 8, 2014 at 7:24 pm

      I learnt to actually reCognise the primary secondary tetiary and quatenary carbon n NItrogEn

      Reply
      • Leah4sci says

        June 8, 2014 at 8:57 pm

        Awesome! Nitrogen tends to throw off lots of students because of the way the carbon atoms are bound/counted. What are you studying right now?

        Reply
        • Johnson says

          June 9, 2014 at 1:34 am

          I’m a first year Pharmacy student………

          Reply
          • Leah4sci says

            June 9, 2014 at 12:43 pm

            Very cool, lots more organic chemistry coming your way 🙂

  52. Elvira says

    February 27, 2014 at 3:25 am

    Thanks Leah really helpful

    Reply
    • Leah4sci says

      March 2, 2014 at 12:22 pm

      You are very welcome Elvira. What did you like most about this trick?

      Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

3 Month MCAT Plan

What does a 3-month MCAT Study Plan entail? Click to Read/Download

3 month mcat study plan leah4sci

Can I help you find a topic?

Download my Free Organic Chemistry Cheat Sheets by clicking on the image below



MCAT Tutorials, Videos & More

MCAT Home Page
Scroll down for orgo resources
MCAT Math Without a Calculator
MCAT Physics
MCAT Chemistry
MCAT Biology
MCAT Biochemistry
MCAT CARS / Critical Reading
MCAT Resources, Tips & More

Organic Chemistry Tutorials & Videos

Orgo Syllabus Companion
- Gen Chem Foundation for Orgo
- Orgo Basics Foundation
- Resonance Structures
- Naming Organic Compounds
- Acids and Bases
- Newman Projections
- Chair Conformations
- Chirality and Stereochemistry
- Fischer Projections
- Intro to Mechanisms
- Alkene Reactions
- Alkyne Reactions
- Free Radical Reactions
- Substitution Elimination Reactions
- Diels Alder
- Aromaticity & Electrophilic Aromatic Substitution (EAS)
- Alcohols
- Oxidation and Reduction
- Acetal / Ketal
- Enolate Reactions
- Proton NMR Spectroscopy

Organic Chemistry Study Guide Cheat Sheets

2023 MCAT Test Dates

Click image below for test + score release dates and more

Radicals: Reactions, Stability Hybridization+

Click for New Tutorial: All about Free Radicals

Free Radicals in Organic Chemistry - Hybridization, Stability, Resonance, Reactions and Mechanism Videos

MCAT Students Come Work With Me

Work with me to figure out exactly what YOU need to ace your MCAT

Option 1: Strategy, tutorials, my help every step of the way in the MCAT Study Hall
[click for details]

Option 2 One-on-one Private MCAT Tutoring

Orgo Students: Come Work With Me

Work with me to ace your Organic Chemistry Course
Option 1: Join me for bimonthly live review/Q & A Sessions, 50+ Hours of Topic-Specific review/practice sessions, direct access to me and so much more... [click for details]

Option 2 One-on-one Private Tutoring

Organic Chemistry Reference Material and Cheat Sheets

Alkene Reactions Overview Cheat Sheet – Organic Chemistry

The true key to successful mastery of alkene reactions lies in practice practice practice. However, … [Read More...]

Click for additional cheat sheets

MCAT Tutorials

mcat math without a calculator 1 play

Introduction To MCAT Math Without A Calculator

While the pre-2015 MCAT only tests you on science and verbal, you are still required to perform … [Read More...]

Click for additional MCAT tutorials

Organic Chemistry Tutorial Videos

KET Keto enol tautomerization reaction and mechanism leah4sci

Keto Enol Tautomerization Reaction and Mechanism

Keto Enol Tautomerization or KET, is an organic chemistry reaction in which ketone and enol … [Read More...]

Click for additional orgo tutorial videos

Copyright © 2023 · Leah4Sci - All Rights Reserved. · Sitemap · Log in